MONDAY, March 5, 2007 (HealthDay News) -- Vaccinating children 2 years of age and older against seasonal flu cuts their rate of infection by almost half (49 percent), U.S. researchers report.
The finding buttresses the government's recommendation that young children under 5 be vaccinated annually against the flu.
However, the shot did not help prevent flu in children under the age of 2 who were not given a booster and follow-up shot, noted researchers at the U.S. Centers for Disease Control and Prevention (CDC).
Still, the flu shot did prove highly effective in older kids, the CDC study found. The effective prevention rate held, despite the fact that during the year of the study -- 2003-2004 -- the vaccine developed for the flu season matched only one-quarter of the circulating influenza viruses, a low "match rate," according to the study authors. Vaccines are developed each year to meet expected new common viruses, but virus strains are unpredictable, so matches between a virus strain and that year's vaccine fluctuate.
"Even though it was a mismatched year, the vaccination was still effective in helping reduce influenza in children, and the study reinforced the importance of vaccinating children each year for the flu," said medical epidemiologist Carrie M. Shuler, who conducted the study when she worked with the CDC. She now works for the Georgia Division of Public Health in Atlanta.
The findings are reported in the March issue of Pediatrics.
Young children are particularly vulnerable to ill effects from influenza, Shuler said, both because of the disease itself, characterized by fever, cough and a runny nose, but also because it increases the risk for other illnesses such as ear infections, pneumonia and urinary tract infections.
"The vaccine reduces the burden on children," Shuler said.
For the study, Shuler and her colleagues compared the vaccination rates of 290 children who were diagnosed with influenza to those of an unvaccinated control group.
Those children who were fully vaccinated -- meaning they had a booster shot and then a follow-up shot more than a month later in the same year -- were 49 percent less likely to get sick, the team found.
They also looked at children who were partially vaccinated, meaning they had only the booster shot but no follow-up shot. In this group, the vaccine proved ineffective for children under 2 -- those children were just as likely to get the flu as were children who had received no vaccination, the CDC team reported. But among older children -- those from 2 to just under 5 years of age -- partial vaccination still reduced their risk for the flu by 65 percent, a significant reduction compared to children who had no vaccination at all.
This difference could be because older children might have already had the flu and so had developed some immunity, Shuler said, while the younger children were very vulnerable to the virus.
"This is an excellent study which confirms previous studies and should be translated to a strong recommendation of the immunization," said Pascal James Imperato, chairman of the department of preventive medicine and community health, State University of New York Downstate Medical Center in New York City.
"It's extremely important that the partial vaccination between 24 and 59 months significantly reduced the influenza infection compared to children in the same age group who had not been vaccinated. This means that even if children are partially immunized, a large number of them will not come down with the infection in a given year," Imperato said. He said that was an important finding, given the resistance among some parents to have children vaccinated and the logistics of going to the doctor's office twice to get shots.
Also, he added, children are on the front lines of spreading the flu because of their relative lack of immunity compared to adults and the fact that children can transmit a virus for as long a week. Adults, on the other hand, are infectious for about three days. "If you want to find out if there's flu epidemic, you first look at children," Imperato said.
Up to 20 percent of the population gets the flu every year, according to the CDC. The disease is primarily spread by coughing, sneezing or touching something with the flu and then touching your nose or mouth. Those at highest risk for complications from the flu include children from 6 months to 5 years of age, pregnant women and people over 50.
The Advisory Committee on Immunization Practices (ACIP), a group of 15 health experts chosen by the U.S. Department of Health and Human Services to provide vaccination guidelines, recommends that children aged 6 to 59 months receive flu vaccinations annually. The first year he or she is vaccinated, a child must have a booster shot and a follow-up shot a little more than one month later. In succeeding years, the child only needs one shot annually.
Vaccines usually become available in the late fall to be ready for the flu season, which generally peaks in January, Shuler said.
There's more on the flu at the CDC.