Study: Protection against the flu begins to decline in a few weeks

Although the US Center for Disease Control and Prevention recommends influenza vaccine delivery at the end of October, a new study published in the US Clinical Infectious Diseases suggests that the effectiveness of the vaccine may begin to decrease in the weeks following administration, which would provide more evidence of decreasing protection in a single influenza season.

Researchers at Kaiser Permanente Northern California have shown that the risk of contracting influenza increases by about 16% every 28 days after vaccination. This means that many people may be less protected at the height of the flu season if they were vaccinated in early September.

The study was conducted over several influenza seasons, from September 1, 2010 to March 31, 2017, and included participants who were vaccinated with the inactivated influenza vaccine (influenza vaccine) and who were then tested for the influenza vaccine. influenza and respiratory syncytial virus (RSV). as a negative control result, by a polymerase chain reaction test.

Instead of a more traditional, "case-insensitive, negative-control" model, this study aimed to determine whether the risk of positive tests for influenza, compared to negative tests, increased with the time elapsed since vaccination .

Results show that flu risk doubles as the season progresses

The study used electronic medical records of 44,959 patients tested positive for influenza in the Kaiser Permanente Northern California health system.

In comparison with vaccinated participants 14 to 41 days before the test, those vaccinated 42 to 69 days before the test had 1.32 (95% confidence interval [CI]According to the authors of the study, the probabilities of testing for influenza range from 1.11 to 1.55). The odds ratio (OR) was 2.06 (95% CI: 1.69 to 2.51) for those vaccinated at least 154 days prior to the test.

No evidence of decline was found for RSV.

According to the authors, most cases were associated with protection against influenza A because this viral strain accounted for 80% of the confirmed cases included in this study.

The authors also included a model that looks at the transfer of influenza vaccination in late November, but they wrote: "Although our findings suggest that a number of influenza cases could be avoided with caution. "

The study highlights the challenge of evaluating the influenza vaccine

Other recent studies conducted in the United States, Spain and the United Kingdom have also shown a decrease in influenza vaccine effectiveness between seasons, but experts caution that the change to the calendar recommendation is a complicated business.

"My informal sense of literature [is] that the suggestion is strong enough that people can be reliably vaccinated a week or two before the start of the influenza season, they would be better protected, "said Marc Lipsitch, PhD, professor of epidemiology at Harvard University wrote a comment on this study: "The most complicated thing is the trade-off between rejecting it and not doing it at all," he said.

Lipsitch said the risk to public health of people who would not get vaccinated during the year could be too high. In his commentary, he stated that a multitude of variables, including antigen drift and circulating strains, make the study of vaccine timing particularly difficult.

"There is an urgent need to find better ways to evaluate influenza vaccines," said Lipsitch in his commentary.

See also:

September 10 Clin Infect Dis study

September 10 Clin Infect Dis comment