Flu, Covid-19, and RSV vaccine effectiveness data for this season is starting to come in. FOI Clinical has pulled out the key numbers you need for patient counseling and end-of-season decision-making. These numbers can help you set realistic expectations with patients and answer the perennial question, "did my flu shot even work?"

Seasonal Vaccine Effectiveness

Studies on this season's vaccine effectiveness are starting to come out. So far, effectiveness looks similar to a typical year, even with the emergence of the new H3N2 subclade K. As always, effectiveness depends on how well circulating strains match the vaccine composition.

Influenza

This season, adults: Canadian researchers estimated how well this season's flu vaccine is working against H3N2, the dominant flu strain circulating this season. Using data from four provinces, they found the vaccine reduced the risk of a doctor visit for H3N2 flu by roughly 40%. Like the United States, Canada predominantly uses egg-based vaccines.

This season, adults: UK Health Security Agency estimated around 75% effectiveness against ED attendance for children and around 60% for adults and older adults. The UK differs from the US in that it primarily uses cell-based vaccines for adults and live attenuated for children.

U.S. data: The US's interim VE estimates are due any time now. They are typically published in January, but this year’s have not been released yet.

Covid-19

This season, adults: An early look at how the updated 2025–2026 Covid-19 vaccine (Pfizer, also known as BNT162b2, targeting LP.8.1) is performing using data from the Veterans Administration. In the first ~4 weeks after vaccination, effectiveness was 57% against ED/urgent care visits and 54% against outpatient visits, which roughly in line with the KP.2 vaccine last year. Uptake is substantially lower compared to last season (15% vs 21% among adults). This is a preprint and hasn't been peer-reviewed yet.

Given the dearth of data for this season, we pulled data from last season as well:

Last season, adults: This study followed over 500,000 matched pairs of vaccinated and unvaccinated US veterans (average age ~71) to evaluate how protection conferred by the 2024–2025 KP.2 Covid-19 vaccine held up over time. Durability was not great. Over an average of about six months of follow-up, vaccine effectiveness was only 17% against documented infection, 21% against ED/urgent care visits, and 20% against hospitalization. Protection dropped steadily over time.

Last season, children: The CDC used data from a nine-state hospital network to measure how well the 2024–2025 Covid-19 vaccine protected kids. Among children aged 9 months–4 years, the vaccine was 76% effective at preventing Covid-related ED/urgent care visits in the first six months after vaccination. For kids and teens aged 5–17, effectiveness was 56%. 

RSV

No studies are available describing RSV product effectiveness for the current season. Data from previous seasons shows good protection against hospitalization for the various available products.

Last season, infants: A study during the 2024-2025 season across 7 medical centers in the US, enrolled 5,029 children under the age of 2 who received outpatient, urgent care, emergency department or inpatient care for acute respiratory infection. Maternal RSV vaccine effectiveness was evaluated in 446 infants <6 months of age, and was found to be 64% effective against medically-attended RSV and 70% effective against RSV-associated hospitalization. Same-season nirsevimab effectiveness was evaluated in 1,765 children, and estimated to be 77% against medically-attended RSV and 81% against RSV-associated hospitalization. Protection against RSV-associated ICU admission was even higher, at 90%.

Last season, infants: In France, 1,270 infants <12 months of age hospitalized for bronchiolitis in 2024-2025 were compared based on RSV status. Estimated effectiveness of nirsevimab against hospitalization for RSV-associated bronchiolitis was 84.9%.

Last season, older adults: This study looked at 1,001 older adults in England admitted to the hospital with acute respiratory infection (comparing RSV+ to those negative for RSV, flu, and SARS-CoV-2) in 2024-2025. The effectiveness of the RSV pre-F vaccine (Abrysvo, Pfizer) was 82.3% against hospitalization for RSV-associated acute respiratory infection. Looking specifically at individuals with immunosuppression, vaccine effectiveness was still quite high, at 72.8%.

Two last seasons, older adults: Researchers studied nearly 7,000 US adults aged 60 or older who were hospitalized for acute respiratory infection in the 2023-2024 & 2024-2025 seasons (RSV+ were cases, and those negative for RSV, flu and SARS-CoV-2 were controls). The vaccines were Arexvy (GSK) or Abrysvo (Pfizer). Vaccine effectiveness against RSV-associated hospitalization was 69% for same season vaccination and 48% for prior-season vaccination. The vaccine was significantly less effective over 2 seasons for immunocompromised adults (30% vs. 67%), and those with cardiovascular disease (56% vs. 80%). Vaccine brand did not make a substantial difference: protection against hospitalization was similar for both Arexvy (GSK, 64%) and Abrysvo (Pfizer, 61%).

Future Vaccine Policy Changes

In related news, The New York Times (gift link) reports that members of the Advisory Committee on Immunization Practices (ACIP), are signaling that mRNA Covid-19 vaccines and also vaccines recommended during pregnancy are their next priorities. Several ACIP members say they believe mRNA vaccines should be pulled from the market, citing concerns about residual DNA contamination. ACIP cannot withdraw vaccines from the market, only the FDA can. However, by rescinding or restricting recommendations, it can potentially reduce clinical use.

The next ACIP meeting is scheduled for February 25–27.