H3N2 Flu Strain: What to Know About the Deadly Outbreak in India

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  • An outbreak of influenza A (H3N2) infections has led to two deaths in India.
  • H3N2 is known to cause more severe illness in young children and older adults.
  • H3N2, which has been the dominant strain circulating in the United States this year, has been associated with more severe flu seasons.

India is monitoring an outbreak of influenza A (H3N2) infections that has led to two deaths, according to the country’s Union Health Ministry.

The deaths were reported in Karnataka and Haryana.

H3N2 is known to cause more severe illness in young children and older adults.

In India, low vaccination rates combined with heavy air pollution worsen the effects of the flu.

India typically experiences two peaks of seasonal influenza: one between November and February and another during monsoon season that runs from June through September.

Dr. Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security and an infectious disease expert, says we see H3N2 circulate every flu season.

And after two historically quiet flu seasons, the flu appears to have returned to its pre-pandemic activity.

“This is a little bit late in the season, but it is still the season and India is in the northern hemisphere. Flu seasons are not all synchronous,” says Adalja.

What to know about H3N2

There are two flu viruses — Influenza A and B — that cause seasonal outbreaks in people.

Currently circulating are the influenza A subtypes H1N1 and H3N2 and influenza B Yamagata and Victoria lineages.

H3N2, which has been the dominant strain circulating in the United States this year, has been associated with more severe flu seasons.

“So far during the 2022-23 flu season in the U.S., about 99% of those hospitalized due to influenza had H3N2 subtype,” says Jennifer Horney, PhD, an epidemiologist and founding director of the University of Delaware’s epidemiology program.

Research shows that H3N2 often causes more severe illness in adults ages 65 and older.

The most common symptoms of influenza A cases include cough, congestion, sore throat, fever, headache, chills, body ache, and fatigue.

Influenza A can be treated with supportive care in addition to antivirals like oseltamivir, baloxavir, peramivir, and zanamivir, says Adalja.

What makes certain flu seasons more severe?

According to Horney, flu seasons driven by H3N2 can include more cases of severe diseases, hospitalization, and death.

This is especially true when vaccine coverage is low or the vaccine is not a good match to the circulating strains, she added.

“A lot of what determines the severity of the flu season has to do with the match of the vaccine to the circulating strain and the percentage of people who are vaccinated each year overall,” says Horney.

Influenza viruses are constantly evolving, and the flu vaccine must be updated annually to remain effective.

Occasionally, the flu virus mutates in a way that reduces the effectiveness of the shot.

According to the Centers for Disease Control and Prevention (CDC), the flu shot is a good match this year — but coverage in the U.S. is lower than usual.

A study conducted in the 2017-2018 flu season found that only 1.5 percent of 65,000 Indian adults ages 45 and older had ever received a flu shot, indicating that vaccine coverage in India is likely very low, says Horney.

In addition, air pollution — which is high in India — can worsen flu symptoms.

“Respiratory function is worse in a high pollution context,” says Adalja. “Any respiratory virus would be worse in those types of situations.”

As a result, the flu’s mortality rate is higher in India — particularly among older adults and young children.

It’s “more than twice the rate per 100,000 people than in the U.S., where we expect between 30,000 and 70,000 deaths from influenza each year,” Horney said.

The bottom line

India is monitoring an outbreak of influenza A (H3N2) that has led to two deaths. Though the uptick in cases is occurring somewhat later in India’s typical flu season, H3N2 circulates every year. This year’s shot is a good match against the circulating strains, and there are effective antivirals that can treat infections caused by H3N2.

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