The flu vaccine has not been a great a match for the predominant viruses this season, preliminary data from the US Centers for Disease Control and Prevention shows – but the season isn’t over, and the estimate could change, the agency told CNN on Thursday.
The flu season has also been atypically mild compared with pre-pandemic flu seasons.
Flu vaccines are based largely on information that lags.
The flu virus is constantly changing. Each February, a panel of US experts helps the US Food and Drug Administration decide which strains to include in the upcoming flu season’s vaccine, to be rolled out in the fall. Each shot can protect against only three or four strains of the flu, so scientists have to make their best guess about which will be circulating in the fall.
The panel bases its decisions on data about the common strains circulating around the globe throughout the year. In some years, the vaccine is a better match to strains that hit the US than others. Even in a good year, the effectiveness of the vaccine is about 60%. Other times, it can be as low as 10%.
This season, based on data from 3,636 kids and adults who got the flu between October and mid-February, the vaccine did not seem to reduce their risk of getting sick from the influenza A (H3N2) viruses that have been predominant. The overall vaccine effectiveness against infection was 16%, which is considered not statistically significant. However, there wasn’t enough enrollment in the study for a reliable understanding of how effective it was.
The details were published Thursday in the CDC’s Morbidity and Mortality Weekly Report.
“I can’t remember an ineffectiveness that was much lower than that,” said Dr. William Schaffner, a professor in the Division of Infectious Disease at Vanderbilt University Medical Center, who did not work on the report. “It was definitely off-target this year.”
The CDC is studying how well the current vaccine protected against severe illness. That’s largely why the agency still recommends that people get the vaccine. Even if it may not keep you from being infected, “vaccination can prevent serious influenza-related complications caused by viruses that might circulate later in the season, including 2009 pandemic A(H1N1) and influenza B viruses,” the study said.
CDC spokesperson Kristen Nordlund also told CNN in an email that the agency recommends the shot “even when protection against one virus is reduced since flu vaccines protect against four different viruses.”
Final data about the effectiveness of this vaccine will be released later this year. Nordlund said people should look at the current information with caution.
“Assessing [vaccine effectiveness[ when there has been little flu circulating and in the midst of the Covid-19 pandemic is complicated and the estimates are not as rigorous as when there is more flu circulation,” Nordlund said.
Schaffner said the study is a good argument for funding ongoing research to develop a better flu shot or even a universal flu vaccine.
“As we say in this business: If you’ve seen one flu season, you’ve seen one flu season,” he said.
A universal flu vaccine, as defined by the National Institute of Allergy and Infectious Diseases in 2018, would be one that provides at least 75% effectiveness for a minimum of one year. Ideally, one could work against influenza A and B and offer up to five years of protection. Seasonal vaccines are more narrowly focused.
Several groups including the NIH have been working on a universal flu shot for decades. It’s needed because a normal flu season brings an estimated 290,000 to 650,000 deaths globally each year, even with seasonal vaccines, according to the World Health Organization.
Get CNN Health's weekly newsletter
Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.
“We need to continue funding the research to create a better influenza vaccine,” Schaffner said. “Going forward, flu is going to continue to appear around the world and make annual epidemics that cause incredible amounts of illness, social and economic disruption, so we clearly need a better influenza vaccine.”
CNN’s John Bonifield contributed to this report