The flu vaccine used in the U.S. for the 2017-2018 season is 36% effective overall, although that rate dropped to 25% in protecting Americans against the predominant strain, H3N2, health officials reported.
In young children six months to eight years, however, the vaccine demonstrated better protection, showing efficacy of 59% against the influenza A and B strains and 51% for H3N2 alone, Anne Schuchat, acting director of the Centers for Disease Control and Prevention told reporters during a Feb. 15 briefing.
She emphasized that the results are interim, so the efficacy rates may change later in the year once the CDC has a better handle on the total number of Americans infected with the flu during the season.
Even though the vaccine is providing low protection overall, health officials urged anyone who has not received the shot to get it.
"There's still time," Health and Human Services Secretary Alex Azar said.
"Getting the flu shot is the same kind of sensible precaution as buckling your seatbelt and there is still time for these protections to make a difference," Azar said.
"The flu vaccination is safe," U.S. Surgeon General Jerome Adams said. "It is still your best defense."
The flu vaccines made for the U.S. market are manufactured primarily by GlaxoSmithKline PLC, Sanofi and Seqirus, which is part of Australia's CSL Ltd. and are formulated with four strains: A/Michigan/45/2015 (H1N1) pdm09-like virus; A/Hong Kong /4801/2014 (H3N2)-like virus; B/Brisbane/60/2008-like virus (B/Victoria lineage); and B/Phuket/3073/2013-like virus (B/Yamagata lineage).
Protein Sciences also markets a quadrivalent recombinant influenza vaccine.
FluMist Quadrivalent, a nasal vaccine from AstraZeneca PLC's U.S. unit MedImmune, also is approved, but was not recommended for use in the U.S. for the current season by a CDC advisory panel.
Better than predicted, but work needed
The 36% overall effectiveness for the U.S. flu shot is better than some researchers and health officials earlier predicted — with some analyses putting the protection as low as 10%, the rate that was also demonstrated in the Southern Hemisphere.
"There is still clearly significant room for improvement," Food and Drug Administration Commissioner Scott Gottlieb said in a statement.
"The FDA is committed to working together with the scientific and medical communities to better protect the public against the flu and apply lessons learned to next season's flu vaccines," he said.
Influenza is different than other viruses because its genetic makeup can change rapidly during the course of a season, Gottlieb explained.
The strains for inclusion in the next season's vaccine are selected well in advance based on the best available information and then it takes several months for the product to be produced, he noted.
The FDA's advisers plan to meet March 1 to recommend the strains for the 2018-2019 season, meaning there is a possibility the strains could change or mutate between then and when the flu starts to spread across the U.S.
"Each year we work hard to predict what strains are likely to be circulating and develop the vaccines around those strains," Schuchat said.
She said there was "no evidence of drift this year" and that "the strains that are circulating are the strains we predicted were likely to be circulating."
"The problem is the vaccine is not working as well as we had hoped," Schuchat said.
She said there are questions about whether the virus that was grown up in eggs to produce the vaccines had mutated or changed in ways that evaded the vaccine.
"That's an issue we are looking at very intensely," Schuchat said.
Flu activity remains high
Schuchat said flu activity in the U.S. remains high and is likely to continue for several more weeks.
Nationwide, influenza-like illnesses are currently mirroring what the CDC observed during the peak of the H1N1 pandemic in 2009, although Schuchat emphasized that does not mean the U.S. is in the midst of a pandemic now.
She noted that hospitalizations are the highest the CDC has ever reported.
The CDC is expected to release updated information on the number of pediatric deaths and hospitalizations on Feb. 16, but as of last week, 63 children had died of the flu.