This year’s flu shot generated relatively robust levels of protection, new data show, with the vaccine’s effectiveness ranging between about 45% and 55%, depending on the age group and the metric studied, such as whether there was protection against any infection or infection that was severe enough to require hospitalization.
The data were presented Wednesday to the Advisory Committee on Immunization Policy, which advises the Centers for Disease Control and Prevention.
Influenza A/H3N2 viruses have predominated this season. The levels of protection generated by this year’s flu shot have been higher than what has been seen in other recent years when H3 predominated.
Even in people in whom flu vaccine often does not induce much protection — people over the age of 65 and people who are immunocompromised — there was a measurable benefit, said Brendan Flannery, an epidemiologist in the CDC’s influenza division.
“When we’ve had H3 viruses for the last 10 years or so, those are groups where it’s hard to show protection,” Flannery said. “It’s at least reassuring that the vaccination in those groups that are often vaccinated and still at high risk, that there was some protection.”
This year’s flu season began remarkably early, with activity peaking around Thanksgiving. About three-quarters of the viruses that have been studied have been H3N2 viruses. That component of the shot is generally the poorest performing part of the vaccine; some seasons vaccine effectiveness studies have shown there was little or no statistically significant protection generated by the shot.
But data from three vaccine effectiveness networks that the CDC mines and from two studies conducted by researchers at Wisconsin’s Marshfield Clinic Research Institute, showed fairly consistent evidence of protection.
The Marshfield work, which was supported by flu vaccine manufacturer CSL Seqirus and by the CDC, estimated vaccine effectiveness in children and adults aged 6 months to 64 years to be 54%. A second study, in children aged 1 to 17 showed vaccine effectiveness of 71% against symptomatic flu infection.
Data from a network of seven children’s hospitals showed the vaccine was 49% protective against being sick enough to either require hospitalization or care in an emergency department. Analysis of data from a second surveillance network put vaccine effectiveness at 46% for people aged 18 to 64 years, and 39% for people 65 and older who turned to an emergency department or an urgent care center for care.
The U.S. findings were in line with vaccine effectiveness data from Canada that were published earlier this month in the journal Eurosurveillance.
Flannery said the viruses that this year’s shot targets are well-matched to those that are making people ill. But additionally, he said, the H3N2 component of the vaccine is strongly immunogenic. “It just works well against the viruses that are circulating.”
Still, though this year’s flu shot performance is good relative to other years, the vaccine effectiveness estimates underwhelmed Sarah Long, one of the members of the ACIP.
“This was the year that we thought there was a very good match,” said Long, a professor of pediatrics at Drexel University College of Medicine. “It’s reassuring somewhat that it’s a higher effectiveness than last year, for instance, or the year before. But it still is, for all of us, disappointing after all of this time and annual immunization. And although we’re saving lives, and saving hospitalizations, we don’t want to lose track of that.”
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