Flu cases on the west coast are on the rise months earlier than normal, prompting health officials to stress the importance of the annual vaccine.

Flu activity typically ramps up in December, hitting its peak in February. But this season has kicked into action earlier this year, with higher positive cases than normal recorded.

Several experts have credited the early seasonal start to a new mutation of the influenza A strain H1N1, which was behind the 2009 swine flu pandemic.

This strain has accounted for 242 of 300 influenza viruses reported by public health laboratories to the Centers for Disease Control and Prevention to date (CDC).

In California, during the past two flu seasons, influenza test positivity hovered slightly above 1% in early October. As of 10 October, that rate is 9.65%, according to data from the state’s department of health.

As of 11 October, Utah has recorded 242 influenza A cases, up from 10 cases this time last year, according to Germ Watch, an Intermountain Health database.

Parts of central Oregon have also seen an influenza A surge.

Influenza vaccines can reduce severe illness and hospitalization, according to the American Medical Association, which recommends flu shots for all people 6 months and older.

The 2026-27 flu vaccines, approved by the FDA in August, ​target H1N1, H3N2 – the dominant strain last flu season – and one influenza B virus.

Scott Hensley, a microbiologist at the University of Pennsylvania, said two of the three components of this year’s vaccine appear to be very well matched to circulating flu strains.

However, the H1N1 mutation might affect antibodies in younger people, and with flu season picking up steam, “it is probably a good idea to get vaccinated early”, Hensley said via ​email.

Promotion of flu shots has been reduced under US health secretary Robert F Kennedy Jr, a longtime anti-vaccine activist who has engineered an overhaul of long-standing childhood vaccine schedules.

Reuters contributed reporting