Feb 22, 2005 (CIDRAP News) The idea that vaccinating schoolchildren is the best way to prevent influenza throughout the US population received a boost last week with the publication of a commentary and a Texas study in separate journals.
In the commentary, two biostatisticians from Emory University in Atlanta argue that the current strategy of targeting high-risk groups has had little effect on the spread of flu and relatively little effect on flu illness and death rates. The authors say a better strategy would be to vaccinate at least 70% of schoolchildren, who are the major spreaders of flu in the general population.
In the Texas study, researchers from Baylor College of Medicine in Houston report that vaccinating about 25% of children in two communities led to significant reductions in flu rates among adults older than 35, relative to three other cities where a conventional flu immunization strategy was used.
The two articles came out the same week that researchers from the National Institutes of Health reported that the increasing flu immunization coverage of elderly people in recent decades has had no detectable impact on flu-related death rates among the elderly. That report prompted the Centers for Disease Control and Prevention (CDC) to issue a statement saying it is still important for older people to get flu shots.
The current CDC strategy stresses flu immunization for groups at highest risk for serious complications of flu, including the elderly, children aged 6 to 23 months, people with chronic illness, pregnant women, healthcare workers, and children on chronic aspirin therapy.
The Baylor study was reported by Pedro A. Piedra and colleagues in Vaccine. They monitored respiratory illness rates among members of one health plan in two communities where children were targeted for vaccination and in three other communities where the usual immunization strategy was used. Baseline rates of medically attended respiratory illness, measured in the 1997-98 season, were about the same in the intervention and comparison communities.
In the following three seasons, children aged 1.5 to 18 years in the intervention communities were targeted for immunization with live-virus intranasal flu vaccine. Between 20% and 25% of the age-eligible children in the two communities were immunized.
The researchers found that illness rates in adults older than 35 in the intervention cities were reduced by 8%, 18%, and 15% in the first, second, and third years of the program, relative to the same age-group in the comparison cities. All the reductions were significant. No consistent drop in respiratory illness rates was seen in younger groups.
The authors calculated that the program eliminated a total of 1,071 medical visits in the two cities over the 3 years.
The commentary making the case for flu shots in children is offered by Ira M. Longini Jr. and M. Elizabeth Halloran in the American Journal of Epidemiology. They write that the existing flu vaccination strategy has had only a small effect on overall flu morbidity and mortality, though vaccination has an important effect on mortality in elderly people.
The authors advocate continuing to immunize high-risk groups at the current level while focusing on using the rest of the vaccine supply on schoolchildren (ages 5 to 18). That approach would provide some direct protection for high-risk people "while greatly curtailing the spread of influenza in the rest of the population by concentrating vaccine in the group most responsible for community-wide transmission."
According to Longini and Halloran, several previously published reports based on mathematical models of flu epidemics predict that vaccinating 70% of children would keep the illness from reaching epidemic levels. The models indicate that vaccinating even 50% of schoolchildren would considerably reduce flu in the general population.
"If the 70 percent threshold can be reached, then high-risk people are protected even if they are not vaccinated," the authors assert. "Since there are about 60 million schoolchildren in the United States, about 42 million doses of vaccine would be needed to get below the epidemic threshold. Thus, even this year we would have had enough vaccine to reduce transmission in the entire country by concentrating on schoolchildren."
Besides the mathematical model studies, the writers cite several other pieces of evidence in support of the strategy. For example, in 1968, vaccination of 85% of schoolchildren in Tecumseh, Mich., led to a 67% lower rate of flu-like illness than was seen in a neighboring town. And in Japan, a policy of vaccinating schoolchildren against flu prevented an estimated 37,000 to 49,000 deaths per year from 1962 to 1987.
Longini and Halloran say they don't know why the United States has never tried the approach they advocate. But they write that until the past few years, public health officials have focused on the direct protection provided by immunization and have not taken the potential indirect effects seriously. Since schoolchildren are not considered a high-risk group, they have been left out of the recommendations.
Longini IM Jr, Halloran ME. Strategy for distribution of influenza vaccine to high-risk groups and children. Am J Epidemiol 2005;161(4):303-6 [Abstract]
Piedra PA, Gaglani MJ, Kozinetz CA, et al. Herd immunity in adults against influenza-related illnesses with use of the trivalent live attenuated influenza vaccine (CAIV-T) in children. Vaccine 2005;23(13):1540-8 [Abstract]
Feb 16 CIDRAP News story "Study: Flu shots in elderly don't cut mortality rate"