Flu Vaccination Guide 2026-2027: Strains, Timing and Who Should Get a Shot

Flu vaccine makers and public-health agencies set a new target each year because influenza viruses mutate quickly. For the 2026-2027 Northern Hemisphere season, the World Health Organization recommended an updated set of three vaccine strains on 27 February 2026, after a newly dominant H3N2 variant reshaped last winter's flu season, according to WHO. In the United States, the Centers for Disease Control and Prevention (CDC) says the underlying recommendation — an annual flu vaccine for almost everyone aged 6 months and older — has not changed, even as a court dispute over vaccine advisory-panel appointments continues in the background.

What's new in this season's vaccine

For egg-based vaccines, WHO recommends an A/Missouri/11/2025 (H1N1)pdm09-like virus, an A/Darwin/1454/2025 (H3N2)-like virus, and a B/Tokyo/EIS13-175/2025 (B/Victoria lineage)-like virus. Cell-culture, recombinant-protein and nucleic-acid vaccines use slightly different reference strains: A/Missouri/11/2025 (H1N1)pdm09, A/Darwin/1415/2025 (H3N2) and B/Pennsylvania/14/2025 (B/Victoria lineage), according to the same WHO recommendation.

The H3N2 component changed because a variant known as subclade K — classified as A(H3N2) J.2.4.1 — was first identified by CDC in the United States in August 2025 and was found to be antigenically drifted from the strain used in the 2025-2026 vaccine, CDC said. The variant, first detected in New York in June 2025, carries mutations that make it unusually effective at evading antibodies built up from past vaccination or infection, researchers told STAT News. It went on to dominate flu activity in multiple countries and contributed to an earlier-than-usual rise in cases in some regions, according to WHO and CDC.

Who should get vaccinated, and when

CDC recommends annual flu vaccination for everyone aged 6 months and older who has no contraindication, under the immunization schedule adopted in July 2025, which it says remains in effect for 2026-2027. For most people the agency advises vaccination in September or October so protection is strong through the winter, though a shot later in the season is still recommended for anyone not yet immunized. The exception is young children who need two doses: CDC says they should start as early as July or August so the series is complete by late October.

• Children aged 6 months to 8 years need two doses at least four weeks apart if they have not already received two or more flu vaccinations before 1 July, CDC says.
• Pregnant women can be vaccinated in any trimester and should receive an inactivated or recombinant vaccine rather than the nasal-spray live vaccine; CDC notes third-trimester vaccination can also protect the infant after birth.
• Adults aged 65 and older should preferentially receive a high-dose, recombinant, or adjuvanted vaccine rather than a standard-dose shot, according to CDC guidance.
• People with egg allergy of any severity may receive any licensed flu vaccine otherwise appropriate for their age and health, CDC says.
• A history of Guillain-Barré syndrome within six weeks of a previous flu shot is listed by CDC as a precaution to discuss with a clinician, not an automatic reason to avoid vaccination.

How well did last season's vaccine work against subclade K

Vaccine effectiveness estimates for the subclade K-dominated 2025-2026 season varied by country and by outcome measured. CDC reported in March 2026 that the season's vaccine reduced the risk of flu-related hospitalization by about 30% in adults and 41% in children and adolescents. Earlier in the season, CDC had cited early England data showing effectiveness against hospitalization of roughly 70-75% for children and 30-40% for adults, while describing US estimates as too early to calculate at that point.

Researchers who study the virus said the moderate numbers reflect how different subclade K is from the strain chosen for the vaccine before the variant emerged, rather than a more dangerous virus. CDC has stated there is no evidence that subclade K causes more severe illness on an individual level; its impact has instead come from the high volume of infections it produced.

Vaccination coverage trends heading into the season

US flu vaccination coverage for the 2025-2026 season was flat to slightly lower than the year before across most groups tracked on CDC's FluVaxView dashboard, as of the dates shown below.

A legal dispute over vaccine policy is unfolding alongside the season

Flu vaccine guidance for 2026-2027 is being issued against the backdrop of litigation over how US vaccine policy is set. In June 2025, the Secretary of Health and Human Services removed all 17 sitting members of CDC's Advisory Committee on Immunization Practices (ACIP) and named replacements. On 16 March 2026, a federal court issued a preliminary injunction in American Academy of Pediatrics v. HHS, staying the appointments of the new ACIP members along with every vote the reconstituted committee had taken since June 2025, according to an analysis by Georgetown University's Center for Children and Families. The stayed votes included a requirement to remove the preservative thimerosal from flu vaccines given to children and pregnant women.

The court found the reconstituted committee lacked sufficient vaccine expertise to represent 'fairly balanced points of view' within the scientific community, according to the ruling cited in the Georgetown analysis. Because of the injunction, CDC says its influenza vaccination recommendations for 2026-2027 continue to follow the immunization schedule adopted in July 2025, before the committee was restructured.