Study: Getting Flu Shot 2 Years in a Row May Lower Protection

March 1, 2013

01 Mar (CIDRAP NEWS) – Experts are puzzled by a new study in which influenza vaccination seemed to provide little or no protection against flu in the 2010-11 season—and in which the only participants who seemed to benefit from the vaccine were those who hadn’t been vaccinated the season before.

The investigators recruited 328 households in Michigan before the flu season started and followed them through the season. Overall, they found that the infection risk was nearly the same in vaccinated and unvaccinated participants, indicating no significant vaccine-induced protection, according to their report in Clinical Infectious Diseases. That contrasted sharply with several other observational studies that found the vaccine to yield about 60% protection during the same season.

In trying to figure out why the effectiveness was so low, the researchers sifted their data in different ways, said Arnold S. Monto, MD, of the University of Michigan, senior author of the study.

“We discovered that if you separated out those that had not been vaccinated the previous year, you got percentages close to what were seen in the major vaccine effectiveness studies,” he told CIDRAP News.

“We were playing with this for a long time, and there was clear interaction of sequential vaccination and vaccine effectiveness, looking at it in a strictly statistical way,” he added. “We felt it had to be separated out.”

The vaccine was found to be 62% effective in those who hadn’t been vaccinated the previous year. That was similar to findings in the other observational studies and also to the results of a recent, rigorous meta-analysis of randomized controlled trials. In contrast, those who had been vaccinated 2 years in a row (before both the 2009-10 and 2010-11 seasons) got no significant protection.

An additional finding was that the vaccine did not seem to protect participants who were exposed to flu in their own household, though the numbers in that arm of the study were small.

Researchers from the US Centers for Disease Control and Prevention and the University of Hong Kong collaborated with University of Michigan researchers on the study, with Suzanne E. Ohmit, DrPH, of Michigan as the lead author.

The findings come amid a growing number of studies that raise questions about flu vaccine effectiveness (VE). They include, among others, last week’s CDC report that this year’s vaccine has worked poorly in elderly people and three recent European studies showing that vaccine-induced immunity in the 2011-12 season waned after 3 to 4 months. Other studies have cast doubt on the long-standing belief that a close match between the vaccine virus strains and circulating strains improves VE.

In an editorial commentary accompanying the Michigan study, John Treanor, MD, and Peter Szilagyi, MD, both of the University of Rochester Medical Center, wrote,

 “As we are currently struggling through one of the most vigorous influenza seasons in recent memory, the apparent failure of influenza vaccine under optimal conditions seen in this study is indeed troubling.”

And Edward Belongia, MD, a Wisconsin clinician-researcher and member of the CDC’s Influenza Vaccine Effectiveness Network, said he was perplexed by the low overall VE in the study, given the approximate 60% protection levels found in studies by the network the same season.

 “I don’t know what to make of it,” he told CIDRAP News.

Other researchers have said that additional studies suggesting a negative effect of prior-year vaccination on flu VE will be emerging in coming months, but they declined to give any details.

AIMING TO DETECT ALL CASES

The researchers used a prospective cohort design in an effort to detect all flu cases in the study group, regardless of whether or not participants were sick enough to seek medical attention.

The team sought to recruit households that had at least four members with at least two children and that received medical care through the University of Michigan Health System, based in Ann Arbor. Out of a target group of 4,511 households, the authors recruited 328, with 1,441 members.

Participants were instructed to report any acute respiratory illnesses throughout the flu season. Individuals with symptoms went to a study site for collection of a throat swab for flu testing. The researchers followed the illnesses to collect data on disease course, including whether the volunteers sought medical attention. Specimens were tested using polymerase chain reaction (PCR).

Among the 1,441 participants, 866 (60%) had documentation of receiving a flu shot for the 2010-11 season, with coverage lower among younger adults and higher in those with high-risk health conditions. Of those vaccinated, 88% received an inactivated vaccine and 12% the live-attenuated vaccine.

During the season, 624 individuals reported 1,028 acute respiratory illnesses, leading to the collection of 983 specimens. Of those, 130 specimens from 125 participants (13%) were positive for flu. By subtype, 45% were influenza A/H3N2, 34% were type B, and 20% were 2009 H1N1. Thirty-two percent of the cases led to medical attention.

Among the 125 people who tested positive for flu, 59% had been vaccinated at least 14 days before their illness onset, long enough for an immune response. The infection risk in the vaccinated people was 8.5% (74 of 866), versus 8.9% (51 of 575) in the unvaccinated individuals.

For more on this article go to Cidrap News.

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