Nancy A. Melville

September 10, 2015

ATLANTA ― The influenza vaccine can protect for 6 months and last throughout the flu season — longer than previous studies have found — and reduce hospitalization in children, according to two studies presented here at the International Conference on Emerging Infectious Diseases.

Protection was consistent for 91 to 180 days after vaccination for 60% of the participants (range, 41% - 72%), reported lead investigator of one of the studies, Jennifer Radin, PhD, from the US Naval Health Research Center in San Diego.

After 180 days, however, there was a significant decrease in effectiveness, to below 11%.

Because the start of flu season varies from year to year, it is difficult to determine the best time to administer the vaccine. Findings from this study suggest that early flu vaccination provides the best protection, Dr Radin's team reports.

"Previous studies found that vaccine effectiveness drops off and does not provide significant protection after approximately 90 to 120 days," Dr Radin told Medscape Medical News. "However, many previous studies used very small sample sizes, and most were done outside of the United States."

She pointed out 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."

 
It is reassuring to know that traditional vaccines have a protection lasting 6 months.
 

And because elderly people are known to be particularly vulnerable to waning vaccine immunity over time, the population studied matters.

"This is why a high-dose influenza vaccine is recommended for individuals older than 65 years," said Dr Radin. "Future studies in these populations looking at declines in influenza vaccine effectiveness over time will be necessary."

Dr Radin and her colleagues evaluated outpatient data collected during four flu seasons ― from 2010/11 to 2013/14. The study involved inactive Department of Defense beneficiaries who attended outpatient facilities in San Diego and Great Lakes, Illinois.

Outpatient febrile respiratory illness surveillance data were used to assess the effectiveness of the vaccine. Age, calendar season, and influenza season were taken into account in the analysis.

Of the 1720 participants, 75% were younger than 25 years and 55% were female. Complete data were available on 1522 of the participants.

The investigators found that vaccine effectiveness ranged from 40% to 69%, depending on the flu season.

This research offers interesting insights into the vaccine's effectiveness, said Cecile Viboud, PhD, from the division of epidemiology and population studies at the National Institutes of Health, in Bethesda, Maryland.

"It is reassuring to know that traditional vaccines have a protection lasting 6 months, which can be useful when immunization campaigns are early and the influenza season is particularly late," she told Medscape Medical News.

"Whether traditional vaccines offer protection that goes beyond 6 months warrants further studies," she added.

Immunizing Children

Dr Viboud presented the second study, which looked at the immunization of children. Data on annual vaccination rates were obtained from the National Immunization Survey and the Behavioral Risk Factor Surveillance System.

Her team found that pediatric flu vaccination was directly associated with a reduction in hospitalizations in children, and indirectly, although more weakly, affected adults.

Flu vaccination rates in children increased from 0% in 2000 to approximately 52% in 2012, which reflected a recommendation for vaccination coverage of children younger than 5 years issued by the US Advisory Committee of Immunization Practices. There were, however, disparities in vaccination rates among states.

Vaccination significantly reduced hospitalization for pneumonia, influenza, and respiratory and circulatory diseases in those 19 years and younger (P < .0001).

In adults 20 to 49 years of age, influenza-related hospitalizations declined, consistent with indirect effects of the vaccine (P < .0001). However, trends were less consistent in older adults.

"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," Dr Viboud said.

"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," she said.

The authors of both studies have disclosed no relevant financial relationships.

International Conference on Emerging Infectious Diseases (ICEID). Presented August 24 and 26, 2015.

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