Introduction

Home health care is one of the fastest growing segments of the U.S. economy and the fastest growing segment of the health care economy (1). Between 1967 and 1980, the number of agencies certified to participate in the Medicare program nearly doubled, from 1,753 to 2,924. Between 1980 and 1985, the number of agencies doubled again, to 5,983.

By 1997, the number of home care agencies has risen to an all-time high of 9,120 (1)

The US Department of Laborís Bureau of Labor Statistics estimates that the number of employees working in home health care has increased from 344,000 in 1991 to about 610,000 in 1995. So-called "formal caregivers" include professionals and paraprofessionals who provide in-home health care and personal care services, and are compensated for the services they provide. The job categories with the highest number of employees are home care aides, registered nurses, social workers, and licensed practical nurses (2).

Developing an Immunization Program for Home Health Care Workers

This chapter provides an outline for starting an immunization program. However, there are no industry standards for immunization programs in home care. Similarly, there are no data or firm recommendations on strategies for vaccine-preventable diseases in home care professionals, although strategies for health care workers, in general, have been detailed (3, 4, 10-12). The recently published Draft Guideline for Infection Control in Health Care Personnel,1997, from the Centers for Disease Control and Prevention includes a discussion of vaccine-preventable diseases. The document also includes a broad definition of health care workers to whom these recommendations apply, noting that "health care personnel, in or outside of hospitals, who have contact with patients, body fluids, or specimens have a higher risk of acquiring or transmitting infections than do other health care personnel who have only brief casual contact with patients and their environment." (11). Another recently published CDC guideline on immunization of health care workers, states that "any medical facility or health department that provides direct care is encouraged to formulate a comprehensive immunization policy for all health care workers (12).

Many organizations have statements on vaccine utilization, most recently the American Medical Association (13). In addition, Federal and State agencies, and accrediting bodies require an employee health program for home care agency employees (15,16). The exact requirements are not specified and the benefits of immunization can be assumed. For example, the Joint Commission on Accreditation of Health Care Organizations (JCAHO) standard for employee health states that "Processes shall be in place to reduce risks for infection in patients and staff members" (16). The Occupational Safety and Health Administrationís (OSHA) Standards are limited to Bloodborne Pathogens and Tuberculosis. The Health Care Financing Administrationís (HCFA) Conditions of Participation for Medicare, the, and Workerís Compensation laws also address health care worker employee health in general terms (15). Finally, the National Association for Home Care can be an important resource in terms of program start-up (1).

A number of factors will influence the final composition and flavor of an immunization program for home health agencies:

(1) During this period of extraordinary growth, no industry standards for immunization have been developed for personnel working in home care settings. This rapid growth is further complicated by various human resource issues, such as high employee turnover rates in some agencies and diverse staff in terms of employee age, professional experience, socioeconomic status, and access to personal health care.

There are no reliable data on turnover overall; only anecdotal information. For example, turnover among home care agencies in Rochester, NY ranges from 5% per year in an agency providing a full range of skilled nursing and home health aide services, to as high as 50% per year in an agency specializing solely in home health aide services.

(2) Home care agencies also tend to be a diverse group in terms of their organization and structure. Home care providers include public and private home nursing agencies, infusion therapy providers, community-based agencies, durable medical equipment (DME) providers, and various combinations of public, government-run and voluntary agencies.

(3) In addition, many home care programs are facility-based; i.e. a part of a hospital, rehabilitation agency, skilled nursing facility or other health system.

(4) Health care worker immunization status is often incomplete (3,4). While there are no data specific to home care, it is safe to assume that home care employees, like other health care workers, will also need to be brought up-to-date with immunization requirements.

(5) There are also scant data on infectious diseases transmission to home care patients (5, 6) or home health care employees (7-9).

Employees working in home care settings are exposed to patients with a variety of vaccine-preventable infectious diseases. Some examples are patients with varicella-zoster, either chicken pox in children or herpes zoster ("shingles") in adults; hepatitis Bí and influenza. It makes good sense for home care agencies to have an immunization strategy as a part of its employee health program. Theoretically, immunization programs should result in safer delivery of care to patients and a safer work situation for employees. Additional benefits of an immunization program include complying with various Federal, State, and Local government regulations, and meeting accrediting body standards (15).

Finally, immunization programs can be cost effective for the home care agency. This is especially important in view of reimbursement constraints placed on Medicare-approved home care providers under the Balanced Budget Act of 1997. As an example, influenza vaccine has been shown to increase employee productivity with employees vaccinated for influenza having fewer absences for acute respiratory illness than unvaccinated employees (14).

Getting Started: Making Policies

Home care programs and agencies should start by developing policies for infectious diseases screening and immunization for their employees. While there are no recommendations that address home care programs specifically, guidelines from the Centers for Disease Control (12), the National Association for Home Care (1), state and local health departments and other home care agencies are good places to start. This includes identifying those employees who should be vaccinated. To start, the core immunization program should include employees who have patient contact. Office staff and other personnel who do not have patient contact can be added once the program has been implemented and is underway. Infectious diseases assessments are an important part of the program and are best done at the time of employment. Employees who do not meet the agencyís immunization requirements can be brought up to date as soon as possible.

The planning process also includes justification for the program. Financial issues will need to be addressed, since the purchase of vaccine, immunization itself, storage of vaccine and record keeping will result in imcremental costs to the agency. However, the impact of the program on employee and patient safety and employee productivity are imortant considerations during the planning phase of the program, some of which are discussed in this chapter.

Once policies are developed, there are many options for managing the agencyís requirements for immunity to infectious diseases. Listed below are 3 possible options for the program. They are listed in order of preference from highest to lowest.

ï Provide vaccinations to employees at agency expense. This option is the most comprehensive approach and is used traditionally by hospitals. This approach also makes sense for home care agencies that are facility-based (i.e. a part of a health facility or health system with its own employee health service). This strategy then manages immunizations uniformly for all employees of the health system and is the most straightforward to manage because vaccination, tracking and documentation are done by the facility itself.

ï Through a contractual arrangement with another health care provider (e.g. Occupational medicine clinic, private physician, other clinic) to provide vaccinations (and other employee health services) to home health care workers.

This is a good option for free standing programs or agencies who are not staffed sufficiently to track and monitor an immunization program.

This strategy requires that the home care agency still maintain oversight of the contracting physician or provider to ensure proper administration of vaccine, documentation, and reporting back to the home care agency (17).

ï Provide only those immunizations that are required by regulation (e.g. Hepatitis B vaccine). Employees receive other vaccines required by the agency as a part of their personal health care through their own health care provider.

This "hybrid" option requires that the agency and employeesí physician work together for compliance and monitoring of completion of the agencyís vaccine requirements and is the most difficult to manage because of the need to vaccinate off-site, track and monitor the program with a large number of outside providers, and the problems inherent in sharing medical records and other information.

Specific-vaccine preventable diseases for home care employees

This chapter outlines two different approaches to an employee immunization program for home health care agencies. The first is a basic program of core, preventive services, consisting of the high priority vaccines recommended by the Centers for Disease Control and Prevention: hepatitis B, influenza measles/mumps/rubella, and varicella vaccines. Also included as a preventive component is tuberculin skin testing, even though it is not a vaccination.

The second strategy involves optional vaccinations that could be considered for implementation after the core program, or may be useful in some home care settings, such as those with low rates of employee turnover or for certain specialty areas, such as HIV or pediatric home care programs.

Core Program

Hepatitis B vaccine: immunity to hepatitis B must be addressed under the Occupational Safety and Health Administrationís (OSHA) Bloodborne Pathogens rule discussed previously. This standard mandates that health care workers, including home care workers, who have exposure to blood, blood products, or needles as a part of their work activity must be offered hepatitis B vaccination at the expense of the employer.

Considerations for home care personnel:

For the purposes of home care personnel, the OSHA rule refers to community health nurses, infusion therapy staff, phlebotomists, and any other staff who come in contact with blood or needles as a part of their work activity.

Considerations for home care patients:

Home care patients, as a group, are not necessarily at increased risk of acquiring hepatitis B in the course of delivery of health care in the home. However, patients in the home may be hepatitis B surface antigen-positive and, therefore, infectious and able to transmit hepatitis B to susceptible home health care workers via blood and needles.

Influenza vaccine: in home care settings, this vaccine should be given to all employees who have patient contact. This strategy will offer protection from influenza to both patients and personnel.

Considerations for home care personnel:

In addition to the protection of patients and personnel, influenza vaccine has been shown to reduce employee absences due to influenza (14). In a study of healthy, working adults, those who received influenza vaccine reported 25 percent fewer episodes of upper respiratory illness than those who received the placebo, 43 percent fewer days of sick leave from work due to upper respiratory illness, and 44 percent fewer visits to physicians' offices for upper respiratory illnesses (14). Therefore, influenza vaccine is an essential, inexpensive component of an infection control program for home care employees. Because of the implications for increased employee productivity, home care agencies should consider offering vaccine to all employees, regardless of their degree of patient contact.

Considerations for home care patients:

Most home care patients will have chronic illnesses, and many will be over the age of 65, putting them at highest risk for influenza-related morbidity and mortality. Home care patients will almost always meet the definition of "high risk" and should be vaccinated with influenza vaccine. While this may not be the direct responsibility of the home care agency, many agencies do offer vaccine to patients in the home, with an order from a physician. Since vaccination of homebound patients may be easily overlooked, assisting patients with influenza vaccination also demonstrates the home care providerís role as patient advocate.

Case 5 below describes another expanded role of home care agencies with regard to influenza vaccine.

Tuberculin skin testing. While not a vaccination, tuberculin skin testing is mentioned here as an essential component of an employee health program for all health care workers, including home health care workers.

Tuberculin skin testing is also a requirement for health care workers under some state and local health codes. It should be done at the time of employment and yearly thereafter, unless otherwise directed by code.

Measles/ Mumps/ Rubella (MMR): These diseases are often considered together because of the trivalent combination of measles, mumps, rubella (MMR) vaccine that is in widespread use in the United States. These diseases are also examples of childhood illness for which home care personnel may be incompletely immunized. In recent years, all states have implemented vaccination laws for school attendance (18 ). However, the requirements vary according to state and, as a result, some home health care workers may not be adequately immunized.

A revised strategy for the use of MMR for health care workers is under review by the Advisory Committee on Immunization Practices (ACIP). The following discussion is based on recommendations in place at the time of this writing.

Measles (rubeola):

Considerations for home care personnel:

Home care agencyís who opt for a broader vaccine strategy should consider measles vaccination for health care workers not previously vaccinated with two doses of measles vaccine after 12 months of age, unless they were born in or before 1957, when the first measles vaccine was introduced.

However, some surveys have shown that 5-10% of health care workers born before 1957 are not immune to measles (19). Employees can be considered immune only if they have documentation of vaccination, a record of physician-diagnosed measles, or laboratory evidence of immunity.

Considerations for home care patients:

Measles can be transmitted to susceptible children, adolescents and adults in home care settings by infected personnel. Studies have demonstrated that health care workers transmit these diseases to patients (19).

Rubella ("German measles"):

There are many reasons why rubella vaccination should be a part of the immunization strategy in home health care settings. One United States initiative has attempted to eliminate the congenital rubella syndrome by widespread vaccination (20). In the past, rubella vaccine programs concentrated on women of childbearing age. As a part of this broader initiative to eliminate the congenital rubella syndrome, the approach to rubella is broadened to include both men and women as vaccine candidates, not just to women of childbearing age. In addition, in many states, immunity to rubella is required as a part of the state health code for health care workers.

Considerations for home care personnel:

While vaccination has decreased the overall risk for rubella transmission in all age groups in the United States by 95%, the potential for transmission in hospital and similar settings persists because 10-15% of young adults are still susceptible (12). Persons born before 1957 generally are considered immune to rubella, according the the Centers for Disease Control and Prevention (CDC). However, studies have shown that about 6% of health care workers (including persons born in 1957 or earlier) do not have detectable rubella antibody.

In home health care settings, both men and women who do not have documentation of having received live vaccine on or after their first birthday or laboratory proof of immunity should be vaccinated. The vaccine is contraindicated in pregnant women.

Varicella: This vaccine is recommended for susceptible adults who have close contact with persons at high risk for serious complications due to varicella. The American Medical Association has recommended this vaccine for all susceptible health care workers (13).

Considerations for home care personnel

Most, but not all, adults born in the United States will have immunity to varicella (chickenpox) by the time they reach the late teenage years. Employees born outside of the United States, in areas such as the Caribbean and parts of Asia, will have a higher rate of susceptibility to varicella.

The varicella-zoster virus, the cause of chickenpox and herpes zoster ("shingles"), can be transmitted to and from susceptible patients and employees in home care settings. Therefore, it makes good sense to vaccinate susceptible employees with varicella vaccine. The number of susceptible employees in most home care agencies is likely to be relatively small. Therefore, it is a good idea to screen employees for susceptibility first, then vaccinate those who are susceptible.

The best way to screen for varicella susceptibility involves taking a history of chickenpox at the time the employee first starts to work for the home care agency. A reliable history of having had chickenpox is a valid measure of VZV immunity. Among adults, 97-99% of persons who say they have had chickenpox are seropositive, can be considered immune and do not require further intervention (12).

A negative or unknown history is not a reliable indicator of being susceptible. Studies have shown that 71-93% of adults who indicate that they have not had chickenpox, or do not know, will actually be seropositive and, therefore, immune (12). This subgroup of health care workers should then be tested serologically to confirm their susceptibility. According to the CDC, serologic testing of personnel with a negative or unknown history of varicella is likely to be cost effective (12). Those personnel who are varicella antibody negative are susceptible and should be vaccinated.

At the present time, the CDC recommends that varicella susceptible health care workers who have been exposed to varicella should be excluded from duty from day 10-21 after exposure. The value of serologic testing of the subgroup of personnel who have negative or unknown histories for varicella is (1) a more targeted and appropriate use of varicella vaccine and (2) avoiding unnecessary furlough of employees with unknown immune status after exposure to varicella-zoster virus.

Considerations for home care patients

Home care workers with either chickenpox or herpes zoster ("shingles") infections can transmit varicella virus to susceptible home care patients, resulting in chickenpox.

Adults: Most adult patients will be immune to varicella, because of past infection. However, a smaller group of susceptible adult patients can acquire chickenpox from employees with either chickenpox or shingles.

Immunocompromised adults in home care settings are usually at higher risk for reactivation of varicella (I.e. shingles), which can be transmitted to adult home care workers who have never had chickenpox.

Children: Although varicella vaccine has recently become a part of childhood immunization recommendations, many children with immunocompromising health conditions and other chronic illnesses are not immune, either because of not having been vaccinated, or by not responding to the vaccine.

Varicella (chickenpox) can be transmitted from health care workers to susceptible children. This transmission can take place from home care workers with either chickenpox or shingles.

Optional Program Components

Tetanus/ diphtheria: This vaccine is recommended as a booster every 10 years. Because of recent outbreaks of diphtheria in Seattle and the newly independent Russian states, it is important for health care workers to maintain immunity.

Considerations for home care personnel

The pre-employment health evaluation is a good time to ensure that this immunization is up-to-date (12).

Use of this vaccine is usually considered a personal health care issue. However, this vaccination makes sense for home care because of the variability in home care environments and the possibility of employee injury in home settings.

Polio: Routine vaccination of adults is not recommended.

Considerations for Home Care Personnel

However, home care workers who are working with pediatric patients who have recently received live oral polio vaccine (OPV) should be fully immunized. This is because shedding of polio virus in feces continues for up to 30 days after vaccination and unvaccinated home care personnel may be at some risk from handling diapers and feces. The risk of transmission from the child vaccinated with OPV to health care workers by feces is very low. However, it is recommended that adults at increased risk of exposure to vaccine poliovirus should be vaccinated with 3 doses of inactivated polio vaccine (IPV) (21,22) .

Hepatitis A: This vaccine is now licensed in the United States. Spread of hepatitis A is related to poor hygiene. It is usually spread via the fecal-oral route and by contaminated food and water. In the United States it is often seen in outbreaks related to spread from food handlers.

There are no food borne outbreaks that are known to have come from food preparation in home care programs or at home by home care personnel. Its use in home care settings is also outside the recommendations of the Advisory Committee on Immunization Practices (ACIP) (23).

Considerations for home care personnel

Hepatitis A vaccine for home health care workers is not an industry standard and the vaccine is not required or recommended for food service workers. While hepatitis A can transmitted via food, food service workers are not at increased risk of hepatitis A because of their occupation (23 ) .

Considerations for home care patients

To date, there are no reports of food-borne hepatitis A in home care patients traced to food prepared as part of a meal program for home-bound patients and this vaccine is generally not recommended for home care patients.

Summary

Employee screening and immunization update at the time of employment makes good sense for home care programs. The employee health strategy helps protect patients and employees from vaccine-preventable illness, and can help maintain employee productivity.

A written strategy for employee vaccination is the best place to start. Table 1 summarizes this recommended strategy. Since state and local health codes will vary in terms of immunization requirements for health care workers, these codes should be reviewed when developing an immunization program. Once this program is in place, additional vaccines as noted in table 2 can be considered.

Table 1

Disease/ Vaccine	Target Employees	Suggested Agency Policy	Rationale
Hepatitis B	Employees with contact with blood or needles	If susceptible, ASAP after employment	Required by OSHA
Influenza	All patient contact personnel	Fall of each year	Employee and patient protection; improves productivity.
Tuberculin skin testing*	All patient contact personnel	On employment and yearly.	Employee and patient protection
Measles (rubeola) *	All patient contact personnel	If employee susceptible or incompletely immunized, use MMR (see text).	Employee and patient protection; update incomplete immunity of employee.
Rubella ("german measles") *	All employees regardless of patient contact	If employee susceptible or incompletely immunized, use MMR (see text).	Employee protection; part of broad, national vaccine strategy.
Varicella	Susceptible employees	If employee susceptible or incompletely immunized, use MMR (see text).	To reduce spread of varicella-zoster virus; a consideration for pediatric home care programs

* may be required by state/ local health codes
 
 

Table 2

Disease/ Vaccine	Target Employees	Rationale
Tetanus/ diphtheria	All employees working in home care settings.	Employee protection; employee health maintenance.
Polio	Employees working with children vaccinated with OPV	Use of inactivated polio vaccine (IPV) to reduce the risk of vaccine-associated paralysis.

 
 
 

Case Studies

These case studies illustrate immunization strategies in several different types of home care agencies. For the most part, these cases illustrate how different agencies manage the core immunization program discussed in the text.

Case 1. This is a New York State-licensed agency which provides professional nursing staff and home health aides to private clients in their homes, and to other home care agencies. The agency has an overall turnover rate among all employees of 45% per year.

The agencyís vaccination policy for employees includes hepatitis B, influenza and measles/ mumps/ rubella vaccines. The agency provides hepatitis B vaccination to all nurses and home health aides, since all of them have potential contact with blood or needles. The agencyís policy is to obtain previous history of hepatitis B vaccination and to screen all employees for hepatitis B markers. Employees negative for all markers are considered susceptible and are offered vaccine through a contracted occupational health service in the area.

The agencyís nursing administrator has the responsibility for monitoring compliance with the vaccination schedule. This is done with a hand-written log of employeesí schedules for the 3 doses of vaccine which triggers a reminder from the administrator to the employee to schedule an appointment with the occupational health service. The nurse-administrator monitors compliance with the vaccine schedule using a faxed note from the occupational health service alerting her to the vaccine being given. Employees who are not vaccinated within 2 weeks of being notified when vaccination is due are placed on suspension and are not assigned cases until they are vaccinated.

This agency also requires vaccination with influenza vaccine yearly, unless the employee has a physicianís letter noting that flu vaccine is contraindicated. Influenza vaccine is given at the agencyís office by a registered nurse employed of the agency.

Tuberculin skin testing is also done at the time of employment and yearly in-house for all nursing and home health aide staff.

Measles/ mumps/ rubella vaccine is provided as discussed in the text. The agency is considering adding varicella vaccine to its program.

Case 2. This large home care agency is certified by New York State and provides a wider scope of services than the licensed agency. They employ a large staff of professional nurses and home health aides. They also subcontract with the agency in case 1 for home health aides.

The employee health program is administered in-house by an administrative nursing care coordinator employed by the agency. Hepatitis B vaccine is offered per OSHA. Influenza vaccine and tuberculin skin testing at the time of employment and yearly are also given by the agency nursing care coordinator or her designate.

Compliance records have been maintained by hand in log books until recently, when the agency acquired the capability to develop its own registry and compliance program on-line. The electronic system was developed specifically for the agency as one component of its medical information system (MIS).

This agency requires that per-diem employees who come from other agencies, usually the agency in case 1, have documentation of hepatitis B vaccination status as a part of its credentialling and quality assurance process.

Case 3. This agency is a privately-owned infusion therapy company that provides a full range of pharmacy, infusion therapy services, and durable medical equipment (DME) to patients at home, in extended care facilities, and via subcontract to other home health providers.

Nursing staff doing infusion therapy are vaccinated with hepatitis B vaccine if susceptible. The agency does not do pre-screening. Instead, screening for hepatitis B surface antibody (anti-Hbs) is done only if employees have been vaccinated in the past. Employees negative for anti-Hbs are re-vaccinated. All other employees are vaccinated without pre-screening.

The agency administers influenza vaccine in-house to all employees annually.

Case 4. The Community Health Network (CHN) is a community-based medical program for HIV care. CHN has a home care program that provides physician oversight and case management to several certified home health agencies. The agency described in case 2 has an employee working as home care coordinator on site at CHN. Infusion therapy services are also provided on site by the agency described in case 3. Home infusion is managed and coordinated by personnel from the agencies described in cases 2 and 3.

As a part of CHNís credentialling process, all employees working in the facility must meet the same health pre-employment health requirements. The check list for employee health requirements for employees of other agencies who work on CHNís site is shown in Figure 1.

Figure 1: Sample Employee Health Checklist for Allied Agencies/ Students

How this form works: employees of outside agencies and students must comply with Community Health Networkís employee health requirements. The form is initiated by the clinic administrator who acts as credentials coordinator .The credentials coordinator works with other agencies or professional schools to obtain this information prior to the employees first day on-site. The coordinator completes the form which is reviewed by the medical director.

Employees not in compliance with these requirements do not work in the facility until they are met. Employees of other agencies who have started, but not completed, a series of hepatitis B vaccinations are cleared for work with patients.

Although influenza vaccination is not required, it is strongly recommended. The agency employee could be given influenza vaccine as a part of CHNís vaccine program or at his/her own agency.

The form has been completed to demonstrate the type of information collected.

Community Health Network

Employee Health Checklist: Allied Agencies/ Students

Employee/ Student name____________________________________

Date of birth______________________________________________

Agency/ School____________________________________________

Contact person/ supervisor______________________________

Phone______________________________________________
 

	Documentation	Result	Date	Comments
Tuberculin	yes	negative	Feb, 1998	Repeat Feb, ë99
Measles	yes	MMR	Sept, 1997
Rubella	yes	MMR	Sept, 1997	---
Hepatitis B	Vaccine series; 2 doses	Pos antibody	3rd dose due- March, ë98	---
Varicella	Yes-	Pos history	childhood
Td	Yes	booster	Sept, 1997
Influenza	none	NA	1997	Needs vaccine- 1998

Review/ sign off:

Administration)__________________________Date____________

Medical Director__________________________Date____________

References

1. National Association for Home Care. 228 Seventh Street, SE

Washington, DC 20003. Also: http://www.nahc.org/

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5. Graham, DR, Nosohusial Infections: Complications of Home Intravenous Therapy, Infectious Diseases in Clinical Practice, 1994; 2: 158-161.

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8.Valenti, W. Infection Control, HIV & Home Care. Caring Magazine: The Journal of the National Association for Home Care (July) 1996; 15: 42-47.

9.Backinger, CL & Koustenis, GH. Analysis of needle stick injuries to health care worker providing home care. Am J Infect Control 1994; 22: 300-306.

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11. Draft Guideline for Infection Control in Health Care Personnel, 1997. Federal Register: September 8, 1997 (Volume 62, Number 173) Pages 47275-47327.

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13. Report of the council on scientific affairs, American Medical Association. Immunization of health care workers with varicella vaccine. Infect Control Hosp Epidemiol 1998; 19: 348-353.

14. Lind, A, Margolis, KL et al. The effectiveness of vaccination against influenza in healthy, working adults. N Engl J Med 1995; 333: 890-893.

15. Bennett, G. Infection Control Components of a Home Care Employee Health Program.. Caring Magazine: The Journal of the National Association for Home Care (July) 1996; 15: 60-63.

16. Joint Commission on Accreditation of Health Care Organizations. Accreditation Manual for Home Care. Chicago: JCAHO, 1995.

17. Centers for Disease Control and Prevention. Improper Infection-Control Practices During Employee Vaccination Programs -- District of Columbia and Pennsylvania, 1993. MMWR: 1993; 42(50);969-971.(Publication date: 12/24/1993).

18. Centers for Disease Control and Prevention. Immunization of adolescents: recommendations of the Advisory Committee on Immunization Practices (ACIP), the American Academy of Pediatrics, the American Academy of Family Physicians, and the American Medical Association. MMWR 1996; 45 (No. RR-13): 1-15.

19. Braunstein H, Thomas S, Ito R. Immunity to measles in a large population of varying age. Am J Dis Child 1990; 38: 183A-186

20. Poland, GA and Nichol, KL. Medical students as sources of rubella and measles outbreaks. Arch Int Med 1990; 150: 44-46.

21. Centers for Disease Control and Prevention. Recommendation of the Immunization Practices Advisory Committee (ACIP): Rubella Prevention MMWR 1984; 33(22);301-310,315-318

22. Centers for Disease Control and Prevention. Recommendations of the immnization practices advisory committee poliomyelitis prevention in the United States: introduction of a sequential vaccination schedule of inactivated poliomyelitis vaccine followed by oral poliomyelitis vaccine. MMWR 1997; 46 (no. RR-3): 1-23)

23. Centers for Disease Control and Prevention. Recommendations of the immnization practices advisory committee poliomyelitis prevention: enhanced-potency inactivated poliomyelitis vaccine--supplemental statement. MMWR 1997 36: 795-798,

24 . Centers for Disease Control and Prevention. Prevention of hepatitis A though active or passive immunization. MMWR 1996; 45 (No. RR-15): 1-30.