Influenza vaccine effectiveness was under 50 percent this season, according to preliminary data from the U.S. Centers for Disease Control and Prevention (CDC), though a smaller study arrived at higher estimates.
Data from the CDC's New Vaccine Surveillance Network showed the vaccines were 49 percent effective against hospitalization or emergency department visits.
The total was higher, 68 percent, for just inpatient admissions.
The network includes seven hospitals across seven states, including Texas Children's Hospital Houston and the Children's Hospital of Pittsburgh.
Preliminary results from a different CDC-run network, the Investigating Respiratory Viruses in the Acutely Ill (IVY) network, showed the vaccines were 43 percent effective against influenza A-associated hospitalizations among adults, including just 35 percent among adults aged 65 and older.
Researchers took data from IVY from Oct. 1, 2022, to Dec. 31, 2022.
IVY is comprised of nearly two dozen sites across the country, including Baystate Medical Center and the Henry Ford Health.
Estimates drawn from a third CDC-run network, the VISION network, showed the vaccines were 44 percent effective against influenza A-associated emergency room or urgent care visits, and 39 percent effective against influenza A-associated hospitalizations.
Estimates were arrived at after examining data from Oct. 15, 2022, to Jan. 24, 2023 for emergency room/urgent care visits and from Oct. 15, 2022, to Jan. 21, 2023, for hospitalizations.
The network has sites in multiple states, including in California.
The estimates from all three networks were reached by comparing vaccination odds among people who had influenza A compared with control patients who tested negative for influenza and COVID-19.
The estimates were adjusted for factors such as age. Unadjusted effectiveness estimates were mostly lower.
CDC officials said in their presentation that the figures showed "consistent influenza vaccine effectiveness" and "substantial protection."
They excluded co-influenza and COVID-19 infections because they believed that could bias the flu vaccine effectiveness estimates.
Dr. Sarah Long, a professor of pediatrics at the Drexel University College of Medicine, and a member of the advisory panel, said that the results were disappointing.
While it's "reassuring that it's higher effectiveness" than the past two years, "it still is for all of us disappointing after all of this time, an annual immunization" brings "such a low protection against hospitalization in adults," she said after the presentation.
She questioned whether officials were examining new vaccines that work better.
"You are asking for the holy grail," Dr. Helen Keipp Talbot, who chairs the panel's Influenza Work Group, responded. Scientists are trying to develop vaccines that trigger better protection but "we're asking the vaccine to do something that the own human immune system has not quite figured out to do," she said, noting that many adults are exposed to the flu throughout their lives but continue to get infected.
The current vaccines are largely the same as the one developed for military recruits decades ago, Talbot said. The vaccines do "prevent a fair number of hospitalizations and hence deaths, so we will continue to use [them] until the holy grail appears," she said.
Researchers examined data from the fall of 2022 through early 2023 to reach the estimates. They are employed by the Marshfield Clinic Research Institute, with the work being partially funded by the CDC.
The Wisconsin research only covered a small number of participants, all within the state. For the child effectiveness estimates, for instance, only 241 children were studied. Like the CDC researchers, the Wisconsin researchers also excluded people who tested positive for COVID-19 and only included controls who tested negative for both influenza and COVID-19. They also didn't count people who were infected by influenza within 14 days of infection.
Multiple influenza vaccines are approved for use in the United States. They are meant to provide "immunization against influenza disease caused by influenza virus subtypes," according to regulators.
Experts try to predict which strains of flu will circulate in the country ahead of time and manufacturers switch up the composition of the vaccines before each season.