Herd Immunity
Thus, the risk‐based approach has not resulted in control of epidemic influenza, nor has it been effective in preventing serious morbidity and mortality.
Herd immunity (protection) is defined as vaccinating one group to reduce the exposure of another. More precisely, it describes a type of immunity that occurs when vaccination of a part of the population (the herd) provides protection to unimmunized individuals. The theory is that, for diseases passed from person to person, it is more difficult to maintain the infection when large numbers are immune. The more individuals who are immune, the lower the chance that a susceptible person will come into contact with an infectious person. Herd immunity is not achieved when an effective vaccine is not available or when vaccines are rejected by a segment of the population. Epidemics of pertussis, measles, and mumps and a resurgence of cases of Haemophilus influenza type b infection have appeared in regions in which immunization has been refused.
Herd immunity has been demonstrated for viral diseases (eg, rubella, measles, and mumps), as well as for diseases caused by bacteria (eg, pertussis, H. influenza type b, and Streptococcus pneumoniae). In the case of rubella, widespread immunization in the United States has resulted in a reduction of the frequency of congenital rubella syndrome from 823 cases per year to 1. By comparison, in the >50% of countries in which rubella immunization is not routinely administered, >100,000 cases of congenital rubella syndrome occur.
Considerable evidence indicates that herd immunity is operative in the control of influenza as well. In Tecumseh, Michigan, >85% of 3159 schoolchildren were given TIV over 4 days and compared to a similar population in the neighboring community of Adrian, where vaccine was not administered. Three times more influenza‐like illness occurred among people of all ages in Adrian than in Tecumseh, demonstrating that immunizing schoolchildren in a community significantly protects the population at large in that community. In 1962, Japanese authorities mandated that all schoolchildren 5–15 years old receive TIV. That practice rapidly and significantly decreased the number of excess deaths attributable to pneumonia and influenza, predominantly among elderly persons. It was estimated that up to 49,000 deaths were prevented annually and that 1 death was prevented by immunizing 420 schoolchildren. In 1986, parents were allowed to refuse vaccination, and the excess death rate rose. In the Temple‐Belton area of Texas, schoolchildren in 2 counties were immunized with LAIV and compared with unimmunized children in 3 counties for the incidence of MAARI among adults >35 years old; even with a vaccine uptake rate of only 20%–25%, indirect protection of 8%–18% of the adults studied occurred. In Russia, Rudenko et al gave either LAIV or TIV to 35%–65% of schoolchildren 7–14 years old. Respiratory illness rates among adults on staff and unimmunized children were inversely related to coverage rates in the immunized children, suggesting that herd immunity occurred. In a day care setting in San Diego, 149 children 24–60 months old and their families received either TIV or hepatitis A vaccine; despite the <45% efficacy of the seasonal TIV used, there was an 80% reduction in the frequency of febrile respiratory illness and a 72% decrease in absenteeism for the school‐age contacts. In another Russian study, 1 dose of TIV was given to school‐age children 3–17 years old (coverage, 57%–72%) in one community (immunized) and none was given to another (comparison). Both communities contained >400,000 people. In the comparison community, the rate of influenza‐like illness in adults >60 years old was 3.4 times higher. In the Maryland SchoolMist Study, LAIV was administered to 40% of a targeted elementary school, and 2 other nonimmunized schools served as controls. Significantly fewer child and adult medical visits for episodes of febrile respiratory illness, fewer over‐the‐counter medications purchased, and fewer days of absenteeism were noted among both children and adult members of families in which children were immunized. In 2000, Ontario, Canada, initiated a universal influenza immunization program for individuals >6 months old. Other provinces used targeted vaccination, and Ontario’s vaccination rate increased 20%, compared with 11% in other provinces. Mortality rates for influenza and influenza‐associated health care use decreased significantly more in Ontario than in the other provinces. Finally, Weycker et al, using a stochastic epidemic model of influenza transmission, clinical illness, and economic costs, estimated the benefits of routinely vaccinating children 6 months to 18 years old against influenza. They predicted that immunizing 20% of this population would result in a reduction in the number of influenza cases by 46% and that 80% coverage would reduce the number of cases by 91%. Similar concurrent reductions were estimated to occur for influenza‐related mortality and economic costs. Thus, significant evidence exists for the operative workings of herd immunity for influenza.