Studies Suggest Flu Vaccine Is Long-Lasting and Reduces Hospitalizations

Two studies presented at the International Conference on Emerging Infectious Diseases show that the influenza vaccine can protect for six months, last throughout the flu season and reduce hospitalization in children.

In one study, outpatient data from inactive Department of Defense beneficiaries who attended outpatient facilities in San Diego and Great Lakes, Ill., was collected during four flu seasons from 2010-11 through 2013-14. Of the 1,720 participants, 75 percent were younger than 25 years and 55 percent were female, and complete data were available on 1,522 of them. Using outpatient febrile respiratory illness surveillance data to assess the effectiveness of the vaccine (taking into account age, calendar season and influenza season), they found that vaccine effectiveness ranged from 40 percent to 69 percent depending on the flu season. According to Jennifer Radin, PhD, from the U.S. Naval Health Research Center in San Diego, protection was consistent for 91 days to 180 days after vaccination for 60 percent of participants, but after 180 days, its effectiveness decreased to below 11 percent. “Previous studies found that vaccine effectiveness drops off and does not provide significant protection after approximately 90 to 120 days,” said Dr. Radin. “However, many previous studies used very small sample sizes, and most were done outside of the United States.” She noted that “other countries have vaccine recommendations different than the United States, and sometimes use different flu vaccine compositions, making comparisons with previous studies somewhat difficult.” This study’s population matters because elderly people are particularly vulnerable to waning vaccine immunity over time, which is why a high-dose flu vaccine is recommended for them.

A second study looked at data on annual vaccination rates of children obtained from the National Immunization Survey and the Behavioral Risk Factor Surveillance System, which found that pediatric flu vaccination was directly associated with a reduction in hospitalizations in children and, indirectly, affected adults. Specifically, the study found that flu vaccination rates in children increased from 0 percent in 2000 to approximately 52 percent in 2012, after the U.S. Advisory Committee of Immunization Practices issued a recommendation for vaccination coverage of children younger than 5 years old. And, vaccination significantly reduced hospitalization for pneumonia, influenza and respiratory and circulatory diseases in those 19 years and younger. In adults aged 20 years to 49 years, influenza-related hospitalizations declined, consistent with indirect effects of the vaccine. “Our results suggest that the childhood influenza immunization program is effective in reducing the severe burden of influenza among children and, hence, the vaccination of this age group should be promoted,” said Cecile Viboud, PhD, from the division of epidemiology and population studies at the National Institutes of Health. “The recent decline in adult hospitalization rates is intriguing. Further research should evaluate whether this is due to herd immunity, declining influenza activity or unrelated long-term time trends.”