Now might be the best time to get your flu shot.
There is mounting evidence that vaccine protection diminishes over the course of a single season since getting the flu shot, meaning an individual vaccinated in early September may be more vulnerable than someone vaccinated in late October when flu season hits.
A recent study published in Clinical Infectious Diseases found that an eight-week delay in vaccination might prevent about nine flu cases for every 1000 persons vaccinated during an average season. These findings backup previous research, including a 2017 study by CDC scientists that found vaccine effectiveness for strains of H1N1 declined up to 11 percent each month after administration.
"We're balancing two things. One is coverage of how many people get the vaccine every year. The CDC recommends everyone over six months of age get the flu vaccine – it's truly our best weapon to protect against this deadly disease. The other thing we're balancing is when is the best time to get the vaccine," said Kunal Rambhia, a University of Michigan graduate student who recently co-authored a paper in Clinical Infectious Diseases titled "Early Bird Gets the Flu." His study summarized 11 studies over the last several years that show evidence for this single-season waning immunity.
Over the past decade, vaccines have become available earlier in the season in efforts to increase coverage, especially as large pharmacy chains started offering them. But because flu season tends to peak in December or later, individuals who got vaccinated during July or August may not be protected the entire season.
Rambhia noted that physicians often want to vaccinate patients who come in for annual or biannual visits during the summer because they might not have the opportunity otherwise when it would be more optimally timed.
"The CDC is encouraging earlier vaccination because of that potentially lost opportunity for people who might not seek the vaccine elsewhere," he said.
The CDC's advisory committee recommends getting the vaccine by the end of October. However, the agency says getting the vaccine in December or later, even if flu activity has already started, is likely beneficial during most seasons. The National Influenza Vaccination Week even occurs in December.
"The recommendation is just so people get the vaccine and have the levels (of antibodies). It takes 10 to 14 days to get the vaccine levels in your system so you're protected. Choosing the end of October is more about making sure people get the vaccine and have the levels, so when the flu hits they have it," said Tanaya Bhowmick, assistant professor of medicine at Rutgers Robert Wood Johnson Medical School.
While the optimal time to vaccinate is before the onset of flu activity, the timing of the season is hard to predict. In 75 percent of the past 35 flu seasons, peak influenza activity did not occur until January or later. Flu activity peaked in February the most times, during 15 seasons, while it peaked in October or November only twice. The historically long and severe 2017-2018 season saw elevated activity nationally through the end of March, with viruses still reported into May.
"Getting vaccinated in August doesn't really make a lot of sense, especially if we don't know how long it's going to last," said Bhowmick.
Though vaccines are available in the summer in many places, only around 2.2 percent of Americans get the flu shot before September, according to CDC data from the past several seasons. October has consistently been the peak month, with 17 percent of people getting vaccinated then.
Delaying the vaccine till later in the fall could lead to higher levels of protection during the peak winter months, but the CDC cautions that waiting should be balanced against risks such as missing the opportunity to get it at all. The flu season might hit a specific community much earlier than it appears nationally. A child in Florida who died of flu earlier this season had not received a flu vaccination.
Even though medical and public health experts urge the vast majority of people to get the shot every year, the CDC estimated flu vaccination coverage among adults decreased by 6 percentage points to 37 percent last season. Persistent misconceptions of the vaccine may be a factor in this worrying trend.
Researchers and health experts are exploring other potential ways to mitigate the effects of waning immunity, such as using a high-dose vaccine for seniors more widely or introducing a two-dose vaccination course during the season. The National Institute of Allergy and Infectious Diseases also has a strategic plan for developing a universal flu vaccine that would protect against multiple subtypes of flu for at least one year.
For now, getting the current vaccine – whether in September or February – is still the best way to protect against this life-threatening disease.
"There's certainly value in getting it later even if you're past the optimal point, just as there's value in getting it early even if it's not necessarily optimal," said Rambhia.
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