Covid-19 nearly eradicated the US flu season, but is influenza gone for good?

As the coronavirus continued to spread across the globe, health experts warned the pandemic could get even worse come flu season with people being hospitalised with both of the contagious respiratory viruses – potentially overwhelming hospital systems.

But instead flu season has been practically nonexistent in the United States and across the world.

As of 20 February, the Centers for Disease Control and Prevention (CDC) has recorded just 1,499 positive flu cases since September. Around the same time last year, the CDC had already logged 174,037 positive cases of influenza.

"This is the lowest flu season we've had on record," the CDC's Lynnette Brammer told the Associated Press. This was based on the 25-year surveillance the agency has done for each flu season.

In normal years, a flu season could cause 500,000 to 600,000 hospitalisations and 50,000 to 60,000 deaths, depending on the severity of the virus that year. But the United States was seeing nowhere near the infections, hospitalisations, and deaths from influenza this current season.

Read more: Not a single case of flu detected in England this year

Several factors have likely contributed to a diminished influenza season amid the coronavirus pandemic, including mitigation efforts like wearing a mask, social distancing, and avoiding large gatherings.

Dr Robert Finberg, an infectious disease researcher and distinguished professor with UMass Medical School, said the Covid-19 virus could also be directly contributing to lower influenza numbers this season.

"Pandemics typically change the ecology of viruses," he said. "For example, in the 2009 pandemic, which was the last flu pandemic, other viruses like respiratory syncytial viruses were decreased because the population was exposed to the pandemic virus."

Being exposed to the virus helped the public "mount an immune response, which sometimes will cause the other viruses to go away or become much less prevalent," he added.

His explanation coincided with a 2020 study in the Journal of Medical Virology where prominent health experts hypothesised that the Covid-19 coronavirus could increase immunity for people against other viruses, such as the seasonal flu. In the study, 8,990 people in New York who tested positive for Covid-19 had a lower likelihood of carrying the flu versus people who did not contract the novel virus.

Experts concluded that the novel virus could potentially be "blocking" the seasonal flu, but more research was needed before making a final determination.

Around the world, other countries were also recording mild flu seasons. In Japan, there were approximately 800,000 flu patients in January 2020, but the country only saw 1,000 flu patients since the second week of 2021, the Wall Street Journal reported. England has not detected a single case of influenza during the first seven weeks of 2021, according to Public Health England (PHE).

Another factor that could be contributing to the low 2020-2021 flu season in the US was that the previous season saw a high number of infections among the public with an estimated 38 million infections, according to CDC data.

"That was a big season," Dr Finberg said. "It would be expected, in a sense, that this year would be a lower season."

When more people become infected by the flu in one season, they develop an immunity that helps them fight the virus for a period of time. This could've potentially led to a mild flu season for 2020-2021, even without Covid-19 circulating and people following mitigation efforts.

Flu season was not over yet, though, and Dr Finberg has implored the public to not get too comfortable with the current infection numbers they were seeing.

"In 2015-16, flu season actually peaked in March, so it does vary," he said. "I'm not certain we are out of the woods yet, so I would urge everyone to be careful."

Experts warned the current mild season, if it continues, should not be what the public expects for the flu in the years to come. Dr Finberg hypothesised the flu could likely come back at normal levels when Covid-19 wanes following the pandemic.

The United States could also experience a severe flu season because so few people have built up antibodies against the virus this current season and mitigation efforts, such as wearing a mask, will be lifted.

Vulnerable communities in future flu seasons could be advised to follow similar mitigation efforts seen during the pandemic like hand washing and wearing a mask to avoid severe illness from influenza.

"I would stress to people not to be too comfortable, not to let their guard down, and to be conscious of getting the flu vaccine in the future," Dr Finberg said. "It could easily come back with a vengeance next year."

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