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Study: Prior flu vaccines may provide residual protection

​​​​​​​​MARSHFIELD, Wis. – Influenza vaccines given in prior years may provide protection against the flu for more than one year, according to a study published online today in the journal Clinical Infectious Diseases.

The study, conducted by flu experts at Marshfield Clinic Research Foundation (MCRF) and the Centers for Disease Control and Prevention (CDC), also reported protection from the flu, due to vaccination, was greatest among people who had no history of flu vaccination in the prior five years.

"We found current season influenza vaccines were effective against the flu regardless of vaccine history, showing that vaccination still offers the best prevention against getting the flu. Study participants who were unvaccinated were the most likely to get the flu," said Huong McLean, an MCRF epidemiologist and the study's lead investigator.

For the observational study, researchers looked at the effect of prior flu vaccinations on current season effectiveness over eight seasons in patients participating in an annual vaccine effectiveness study at Marshfield Clinic. Researchers collected samples from patients who went to the doctor with flu-like symptoms and calculated vaccine protection by comparing the flu rate in vaccinated and unvaccinated people.

"Other studies have looked at the effect of vaccination in the previous season, but this is the first study to look back at the five-year vaccination history and see how it affects vaccine protection in the current season," McLean said.

Researchers found adults had similar levels of protection if they received the flu vaccine in the current season, the previous season or both seasons. They also found that vaccine effectiveness was 27-41 percent higher for adults who were vaccinated for the first time in the past five years compared to those who got the vaccine almost every year.  She said reasons for the increase in effectiveness is not yet clear and more research is needed to understand how repeated flu vaccination affects the immune system.

"This study raises questions about a complicated and important issue that needs to be examined further, preferably in a clinical trial," McLean said.

"Although this study raises some new questions, it also confirms that the flu vaccine is effective when given every year and it remains the best strategy for preventing serious flu infections," McLean said.

She also said getting vaccinated once every five years might provide better protection for that season but it would be a bad strategy during the other four seasons. Some flu seasons are mild, some are severe and it would be risky to skip vaccination.  

The CDC recommends annual flu vaccination for all adults and all children who are at least six months old. The vaccine is safe and effective.

This study was conducted using data collected from Marshfield Clinic for the U.S. Flu Vaccine Effectiveness Network. MCRF was the first Network site to receive CDC funding to study flu vaccine effectiveness and was the sole provider of this information for four flu seasons beginning in 2004-05.

The complete report is available at​

​​The Marshfield Clinic system provides patient care, research and education in more than 50 locations in northern, central and western Wisconsin, making it one of the largest comprehensive medical systems in the United States.

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