Protection of health care workers against influenza infection is potentially beneficial for three reasons: (1) prévention of influenza illness and the costs associated with its medical care; (2) reduction of absenteeism; and (3) prevention of nosocomial influenza infection of patients. It has been difficult to prove that the currently available influenza vaccine achieves any of these benefits. This article reviews studies that attempt to demonstrate beneficial effects of influenza immunization for health care workers and the reasons for the current recommendations.
DOES INFLUENZA VACCINE PROTECT HEALTH CARE WORKERS AGAINST INFECTION?
Healthy adults are susceptible to influenza infection with its fever and significant respiratory complications. The likelihood of infection as determined by seroconversion) varies greatly among health care workers. Wilde et al.1 found that 8.9% of unimmunized residents and nurses became infected with influenza A and 5% with influenza B during a 3-year period, but yearly rates varied from 2.6% to 20% for influenza A and from 0% to 8% for influenza B. Elder et al.2 demonstrated an incidence of 23% among health care workers during one season.
However, it has been difficult to establish that immunization of a population not likely to experience significant lower respiratory tract complications is worth the cost and potential adverse reactions of the vaccine itself. Variables that may affect the ability to demonstrate such vaccine effectiveness include (1) the end point studied (eg, clinical illness compatible with influenza, laboratory evidence of influenza infection, or absenteeism); (2) the severity of the influenza outbreak during the season under study; and (3) the antigenic "match" between the strain(s) identified during the outbreak and those included in the vaccine being used.
Weingarten et al.3 were unable to demonstrate a significant difference between immunized and unimmunized hospital employees when comparing incidence or severity of influenza-like illness or absenteeism due to illness. No attempt was made in their study to document infection by serology or culture. In comparing both rates of illness and laboratory evidence of infection among vaccinated and unvaccinated members of a hospital's staff, Feery et al.4 detected significantly increased rates of isolation of influenza A from those who had not been immunized. The incidence of respiratory tract illness was not reduced by immunization, however. Similarly, Hammond et al.5 found that protection against serologie evidence of infection among medical students and hospital staff in Australia was not accompanied by a reduction in reports of respiratory symptoms.
Wilde et al.1 recently conducted a randomized, blinded study of 180 vaccine recipients and 179 unimmunized hospital-based health care workers during three winter seasons. Vaccine efficacy as evaluated by serologie surveillance was high (88% for influenza A and 89% for influenza B). Among the unvaccinated participants with serologie evidence of influenza infection, febrile respiratory illnesses were significantly more frequent and lasted longer than those among individuals who were unimmunized but had no serologie evidence of infection. Immunized individuals had fewer cumulative days of febrile respiratory illness during the influenza seasons and fewer days of absenteeism due to respiratory illness than did unimmunized individuals as a group, but the differences were not statistically significant.
An important point made by these investigators is that, compared with other groups of adults who suffer from influenza-associated respiratory infections, health care workers (most participants in this study were house officers) appear to be less likely to stay home from work during their illness. Although their stoicism may be appreciated by their colleagues in the short term, it is likely to put both their coworkers and their patients at risk for infection.
These studies "show that health care workers are at risk for influenza infection and that influenza vaccine provides significant protection, but it is difficult to document that this protection has a significant impact on the frequency or severity of illness, or on absenteeism. These data contrast with the results of a recent study by Nichol et al.,6 who demonstrated a 0.5-day reduction in absenteeism in a randomized, controlled trial among immunized healthy adults who were not health care workers.
It is likely that reductions in respiratory illnesses are so difficult to demonstrate in healthy health care workers because illnesses due to yearly outbreaks of influenza are diluted by illnesses due to the other respiratory viruses. It may also be, as Wilde et al.1 speculate, that reluctance on the part of health care workers to stay home from work, even when they are ill, means that a reduction in respiratory symptoms does not result in a reduction in absenteeism.
Despite uncertainties regarding the benefit of yearly influenza immunization for health care workers themselves, the Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention recommends that, "Administrators of all health-care facilities should arrange for influenza vaccine to be offered to all personnel before the influenza season." The recommendation .further states that personnel should be "strongly encouraged to receive vaccine," particularly those who care for high-risk patients, including those in intensive care and medical-surgical units.7 The Committee on Infectious Diseases of the American Academy of Pediatrics recommends influenza immunization for "all health care personnel in contact with pediatrie patients in hospital and outpatient care settings."8
Although yearly influenza vaccination is offered to nursing and medical staff by health care facilities in the United States, Canada, and many European countries, compliance is less than 10% at many institutions.9 Reasons for health care workers' rejecting immunization include confidence that they will be able to ward off infection on their own, perception of low risk of exposure, doubts about vaccine efficacy, and concern about adverse reactions. However, some studies demonstrate that it is possible to enhance immunization among health care workers through education and by providing convenient sites for immunization each fall.9
PROTECTION OF PATIENTS AGAINST INFLUENZA INFECTION
Nosocomial influenza infection puts hospitalized infants and children at risk for excess morbidity and mortality and prolonged hospital stays.10'12 During the 1970s and early 1980s, enhanced availability of practical diagnostic techniques allowed recognition of specific viral agents among pediatrie patients. Studies by Hall and Douglas13 and others14 provided evidence that rapid diagnosis of respiratory syncytial virus infection among hospitalized infants and children, coupled with an understanding of how that virus is transmitted, could lead to effective prevention methods for hospitalized patients. Isolation policies on pediatrie units have improved since 1975 when Hall and Douglas demonstrated that 12 of 13 infants who had intercurrent fever during hospitalización had acquired nosocomial influenza infection.10 Nosocomial infection remains a significant risk. Serwint et al.11 described the impact of influenza on the morbidity and mortality of hospitalized pediatrie patients: 8% of infected patients acquired their infection after hospitalization. Nosocomial influenza has been documented in children with cancer15 and in neonatal intensive care units,16,17
Prevention of nosocomial influenza among pediatrie patients might be enhanced by prevention of infection among health care workers. This approach would be especially important for young hospitalized infants because the currently licensed vaccine is poorly immunogenic and not recommended for infants younger than 6 months. In addition, Kempe et al.15 have provided evidence that a level of influenza antibody that correlates with protection in other populations does not guarantee protection in immunocompromised children with cancer. If immunization of these patients often fails to provide protection, perhaps they could be protected by immunizing the individuals who care for them.
Although there are no prospective studies testing the effectiveness of immunizing health care workers to prevent nosocomial infection in pediatrie patients, some studies have suggested prevention in older adults. One study from Scotland18 demonstrated that immunization of elderly patients in a long-term-care hospital had no impact on patient mortality or respiratory illness. However, patient mortality and the proportion of patients with respiratory influenza-like illness were significantly reduced in units where immunization was encouraged among health care workers, of whom 61% received vaccination. In this first study, no attempt was made to document influenza infection using serology or culture.
The same investigators extended their study in a randomized, prospective manner to 20 longterm-care geriatric hospitals.19 They found that the mortality rate was 22% in hospitals that did not offer influenza immunization to their staff and 13.6% in hospitals that did. However, the reduction in mortality was not associated with a reduction in documented influenza infection among patients whose caretakers were vaccinated, so the relationship between immunization and reduced mortality was uncertain.
Influenza is an important cause of morbidity among infants and young children. Neuzil et al.20 recently demonstrated, in a Medicaid population, that excess hospitalization rates due to influenza occur disproportionately among infants younger than 12 months, but immunization of infants younger than 6 months with the inactivated influenza vaccine is not recommended. Even among high-risk children for whom immunization is recommended, there is poor use of vaccine.11 Adult vaccine efficacy rates of 70% to 90%1<21 suggest that immunization of health care workers would reduce exposure of hospitalized infants and children. Whether that protection would be significant in the face of a communitywide outbreak is untested.
What are the costs of immunizing health care workers? At our hospital during the fall of 1999, the cost of vaccine alone was $2.75 per dose, with additional costs in nurses to administer it and supplies totaling approximately $1,500 to immunize 7,091 individuals. Some hospitals have enhanced participation by providing incentives and dedicating staff immunization carts that travel through the hospital each fall. Although these efforts are costly, so are nosocomial infections. At one hospital in South Dakota, 6 adult cases of nosocomial influenza infection resulted in an added patient cost of $24,300 in 1993.22 That hospital instituted an aggressive program resulting in a staff immunization rate of 68% to 72% during the successive four seasons. Nosocomial infection was reduced to no more than 1 case per year.
IS THERE AN ARGUMENT AGAINST INFLUENZA IMMUNIZATION?
Fear of adverse reactions prevents some staff members from accepting immunization. When adverse reactions have been prospectively tabulated among health care providers, only minimally troublesome effects have been detected. Weingarten et al.3 found no difference in reports of fever, arthralgias, rash, wheezing, or induration at the injection site between 54 immunized hospital employees and 88 individuals who received placebo. Erythema (11%) and pain at the injection site (51%) were significantly more frequent among vaccine recipients.3 Wilde et al.1 questioned vaccine and placebo recipients during 3 successive years and found no significant adverse events among either group. Only local pain and swelling emerged as frequent adverse reactions.
Some earlier investigators suggested that yearly inactivated influenza vaccine might lead to a reduced immunologie response from infection during successive years.23 However, several groups have since demonstrated that neither immunologie response to vaccine nor protection from infection is significantly reduced by yearly influenza immunization.1,21,24
Although it is difficult to prove that yearly immunization of health care workers reduces illness or absenteeism or leads to significantly less nosocomial illness, advisory bodies including the Committee on Infectious Diseases of the American Academy of Pediatrics and the Advisory Committee on Immunization Practices have recommended its use. Significant reduction in illness compared with that seen in unimmunized individuals has not been demonstrated among health care workers, but protection from influenza infection as determined by seroconversion during influenza season has been documented. Infected health care workers who continue working despite symptoms, or who have asymptomatic infection, put patients at risk. Because many hospitalized pediatrie patients are unimmunized, immunologically compromised, critically ill, or all three, the current recommendations make sense, even if the benefits of immunization are difficult to prove.
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