Influenza A(H3N2) is the dominant flu strain circulating right now, accounting for roughly two-thirds of subtyped influenza A cases in recent CDC surveillance data. Influenza A(H1N1) and influenza B (Victoria lineage) are also circulating, though at lower levels. Here’s what that means for you and how these strains differ.
Which Strains Are Circulating
CDC surveillance breaks flu activity into three main categories: influenza A(H3N2), influenza A(H1N1), and influenza B. In recent weeks of the current season, the numbers look like this:
- A(H3N2): ~3,100 positive specimens, making it the clear leader
- Influenza B: ~1,030 positive specimens combined (both Victoria lineage and lineage-unspecified cases)
- A(H1N1): ~560 positive specimens
H3N2’s dominance is consistent with early-season patterns reported by the CDC, where it represented over 70% of subtyped influenza A viruses. Influenza B activity tends to stay low early in the season and pick up later, which appears to be happening now with B cases making up a meaningful share of detections.
B/Yamagata Is Gone
If you remember flu vaccines covering four strains instead of three, that’s because they used to include two influenza B lineages: Victoria and Yamagata. The B/Yamagata lineage hasn’t been detected anywhere in the world since March 2020. The FDA formally recommended removing it from vaccines starting with the 2024-2025 season, and flu shots are now trivalent, covering H1N1, H3N2, and B/Victoria only. This isn’t a temporary decision. The scientific consensus is that B/Yamagata no longer poses a public health threat.
How H3N2 and H1N1 Feel Different
Both strains cause classic flu symptoms, but research comparing seasons dominated by each subtype has found some consistent differences. Fever is the most common symptom regardless of strain, showing up in about 80% of confirmed cases. Beyond that, the experience can diverge.
H3N2 tends to produce higher fevers. In a large comparison study, 60.5% of H3N2 patients had temperatures at or above 38°C (100.4°F), compared to 47.2% during an H1N1-dominant season. H1N1, on the other hand, is more likely to come with body aches, cough, and sore throat. Myalgia (that deep muscle soreness) appeared in nearly 72% of H1N1 cases versus 48% for H3N2, and cough was reported in 64% of H1N1 cases compared to 41% for H3N2.
The reassuring finding: rates of serious complications like pneumonia and hospitalization were similar between the two subtypes. Upper airway complications such as bronchitis, sinusitis, and tonsillitis also showed no significant difference. So while the day-to-day misery might feel a bit different depending on which strain you catch, neither one is dramatically more dangerous than the other in most people.
Hospitalization Activity This Season
Hospitalizations are climbing along the expected seasonal curve. The CDC reported 1,665 lab-confirmed flu hospitalizations in a single recent week, with a cumulative total building since October. H3N2 is driving the majority of those admissions, which is typical for H3N2-dominant seasons. H3N2 has historically been harder on older adults and young children, the groups that consistently account for the highest hospitalization rates.
How Well the Vaccine Matches
The 2025-2026 flu vaccine was designed to target the strains currently in circulation. The trivalent formula includes an H1N1 component, an H3N2 component, and a B/Victoria component. Interim vaccine effectiveness estimates from this season’s data in Germany found 31% effectiveness against any influenza in outpatient settings and 69% effectiveness against influenza requiring hospitalization. Protection appeared strongest against influenza B, with lower effectiveness against influenza A, particularly in adults under 60.
Those numbers might sound modest, but they reflect a consistent pattern with flu vaccines: even partial protection significantly reduces your risk of ending up in the hospital. A vaccine that’s 69% effective at preventing hospitalization is still doing meaningful work, especially for people with underlying health conditions.
What About Bird Flu?
H5N1 avian influenza is not one of the strains “going around” in the way seasonal flu is. It’s widespread in wild birds and has caused outbreaks in U.S. dairy cows and poultry, with 71 human cases reported since February 2024. Nearly all of those cases occurred in dairy and poultry workers with direct animal exposure. There is no known person-to-person spread of H5N1 at this time, so it’s not circulating in communities the way H3N2 or H1N1 are. The CDC monitors for any crossover into its regular flu surveillance systems, and so far the numbers remain small and confined to occupational exposures.
Seasonal Timing and What to Expect
During the 2024-2025 season, H1N1 and H3N2 circulated at roughly equal levels, with influenza B staying low until later in the season. The current season is following a similar early pattern, with H3N2 taking a stronger lead. Flu seasons in the U.S. typically peak between December and February, though activity can stretch into March or April. The late-season uptick in influenza B cases is worth watching, as B tends to linger after influenza A activity has started to decline.