Flu season has arrived earlier than usual in the United States, with the West Coast seeing some of the first notable increases.
In a typical year, flu activity begins building in October and November. The initial rise usually appears in the South before moving northward and then westward.
This year, however, data from the Centers for Disease Control and Prevention (CDC) show a markedly different pattern.
Flu activity began increasing during the week ending August 8 and has continued to climb gradually but consistently.
Between September 19 and October 3—the latest period for which data are available—the flu positivity rate, meaning the share of tests detecting the virus, rose from 2.5 percent to 3.6 percent. During the same period last year, the rate was just 0.6 percent.
Flu-related emergency department visits also doubled over those two weeks, increasing from 0.3 percent to 0.6 percent of all ED visits. At this point last year, they represented 0.2 percent of visits.
By region, official figures indicate that flu levels are rising along the West Coast—typically the last part of the country to experience a major seasonal surge.
Dr Helen Chu, a respiratory infections specialist at the University of Washington in Seattle, said: “This year, it seems like we’re having a flu surge very, very early.”
Official data indicate that flu cases are increasing earlier than usual, particularly in the western US. The image above shows someone receiving a flu vaccine.
She told HuffPost: “Certainly in the Pacific Northwest and… in California as well, there has been an early rise in flu cases.”
The reason for the unusually early increase remains uncertain. However, experts have pointed to the dominant circulating strain, H3N2 subclade K—a mutated version of last year’s “super flu.”
Dr Amesh Adalja, an infectious diseases expert at Johns Hopkins, told Prevention: “The leading explanation is [we have] a new variant whose mutation lets it get past the antibodies people built from past infections and vaccines.”
He said the change could leave more people vulnerable to infection, allowing the virus to “spread sooner” than it has in previous years.
Experts also do not yet know why the West Coast is seeing an earlier rise this year. They noted, however, that the region where flu begins spreading first can vary from one year to the next.
An early start does not necessarily indicate that the flu season will be more severe. Still, the increase has prompted experts to urge people to get vaccinated without delay.
Dr Danielle Zerr, who leads the pediatric hospital division at Seattle Children’s, told the Washington Post: “[Flu cases] are about two months early [in this area].”
“The way flu is circulating now, you wanted that vaccine in you two weeks ago, if not a little before that,” she added.
The CDC recommends annual flu vaccination for everyone aged six months and older, apart from “rare exceptions.”
The vaccine is especially recommended for people at higher risk of hospitalization or death from the flu, including adults aged 65 years and over and those with underlying health conditions that weaken their immune systems.
The graph above tracks the rising flu positivity rate—the proportion of swabs that detect the virus—in the US.
Studies show flu vaccines in previous years have been between 35 and 60 percent effective at preventing a serious illness from the flu.
Unlike last year, experts say that the shot has been updated to trigger protection against the H3N2 subclade K variant.
Influenza, commonly known as the flu, is one of the most common and highly contagious respiratory infections, impacting around eight percent of the US population each year.
The infection can cause symptoms such as fever, cough, sore throat, stuffy nose, headaches and fatigue, and can lead to more serious complications including pneumonia and influenza-associated encephalopathy (IAE), a severe brain dysfunction condition.
While most people recover within a week, some may develop complications such as pneumonia, which can be life-threatening and require medical treatment.
It’s for this reason that doctors recommend an annual flu vaccination, which provides protection for six to eight months, to help lower the chances of being infected.
In terms of other diseases, wastewater surveillance shows that COVID cases are ‘low’ nationwide and have remained flat for the past few weeks, while cases of respiratory syncytial virus, a virus spread via air droplets that causes flu-like symptoms, are ‘very low.’
This year’s flu vaccine was updated to trigger protection against both the H1N1 and H3N2 strains circulating in the US.
