Does flu vaccine wane during season?

CDC still advises vaccination by October

Getting an early start on influenza vaccination may not be the best strategy. Three studies from Europe have raised new questions about waning immunity after vaccination — within the same flu season.

The Centers for Disease Control and Prevention does not recommend changing the start date of fall influenza vaccination campaigns, but the agency is seeking more information on the issue of waning immunity and vaccination timing, says Lisa Grohskopf, MD, MPH, medical officer with CDC’s Influenza Division.

“We don’t know when the season is going to start, so we don’t know when to tell people to start vaccinating,” says Grohskopf, who notes that influenza outbreaks can begin as early as October. “How early is too early and how late is too late depends on the season.”

Currently, CDC still recommends starting influenza vaccination “soon after vaccine becomes available, if possible by October.”

European researchers have been finding evidence of waning immunity. For example, a British study found that vaccine effectiveness against H3N2 was 43% for the first four months of the 2011-2012 season (October to January) and just 17% for the final three months of the season.1

However, because of the small sample size compared to what is needed for vaccine effectiveness studies, the confidence intervals are large, leaving some uncertainty in the findings, Grohskopf notes. In the British study, the confidence interval for effectiveness early in the season ranges from -34% to 75% and the confidence interval for later in the season ranges from -24% to 45%.

There are other considerations as well. If vaccination starts later, it may be difficult to manage large vaccination programs, whether they are employee-based or public campaigns, says Grohskopf. The Advisory Committee on Immunization Practice, a CDC expert advisory panel, continues to recommend universal vaccination. About 145 million doses were expected to be available this flu season.

“There are concerns that if the programs start later, you miss opportunities to vaccinate,” says Grohskopf. “Deferring to later makes it more difficult to get [the vaccination program] done.”

Effectiveness low for elderly

Overall, the effectiveness of the flu vaccine has gained more scrutiny. In this flu season, the vaccine was only 47% effective against influenza A strains for people under 65, but 67% effective against influenza B, the CDC reported. The combined effectiveness of the trivalent inactivated vaccine, the most common version, was 56%.2

“Certainly, we’d all like to see a better vaccine,” Grohskopf says. “Having 50% or 60% [protected] is better than not having any protection.”

The vaccine performed even more poorly among the elderly this season. The overall effectiveness of the trivalent inactivated vaccine was 27% for people 65 years of age and older, but the effectiveness was only 9% against the H3N2 strain, which is known to cause more severe disease.

Older people may not mount an adequate immune response, particularly to H3N2, influenza experts say. However, they also have a higher risk of hospitalization and death from complications of influenza and should still be a priority group for vaccination, CDC researchers say.

In the 2013-2014 flu season, an unprecedented number of types of vaccines will be available, including two that use a new, non-egg-based technology. (See related article on p. 40.) ACIP, the federal advisory panel, approved the use of a quadravalent vaccine, which will protect against two A strains and two B strains. The trivalent vaccine protects against two circulating A strains and one B strain.

Fluzone, a high-dose inactivated influenza vaccine, was approved by the U.S. Food and Drug Administration for people 65 and older. However, the CDC has not stated a preference for particular types of flu vaccine for different populations.

“The messages are going to be more complicated next year than ever,” says William Schaffner, MD, chairman of the Department of Preventive Medicine at Vanderbilt University in Nashville, TN, and past president of the National Foundation for Infectious Diseases.

People receiving the trivalent vaccine should not consider it to be inferior to the quadravalent vaccine, he says. More research is needed to determine which vaccines are more effective for different age groups and patient populations, he says.

References

1. Pebody RG, Andrews N, McMenamin J, et al. Vaccine effectiveness of 2011/12 trivalent seasonal influenza vaccine in preventing laboratory-confirmed influenza in primary care in the United Kingdom: evidence of waning intra-seasonal protection. Eurosurveillance 2013; 31:18 Available at www.eurosurveillance.org/ViewArticle.aspx?ArticleId=20389. Accessed on February 22, 2013.

2. Centers for Disease Control and Prevention. Interim adjusted estimates of seasonal influenza vaccine effectiveness — United States, February 2013. MMWR 2013; 62;119-123.