Disease specialists have urged the authorities to prepare for a so-called twindemic this winter, where COVID-19 and seasonal influenza outbreaks could increase pressure on health services and cause additional deaths.

The term twindemic - which is a portmanteau of "twin" and "pandemic" - was coined last year, when experts first warned of the dual threats of flu and the novel coronavirus.

Flu outbreaks were, in fact, kept to historically low levels last winter in many parts of the world, most likely due to COVID-19 lockdowns, the wearing of masks, hand washing, and social distancing.

But experts are sounding the alarm again in the wake of many governments loosening COVID-19 restrictions.

"We are likely to see the flu for the first time in any real numbers, co-circulating with COVID-19," Jenny Harries, chief executive of the United Kingdom Health Security Agency, told Sky News. "The risks of catching both together still remain. And if you do that, then early evidence suggests that you are twice as likely to die from having two together, than just having COVID-19 alone."

A 2020 Public Health England hospital study of seriously ill patients found 43 percent of people with both flu and COVID-19 died, compared to 27 percent of those who tested positive for COVID-19 alone. Of patients in the study who only had the flu, 4.8 percent died.

Last week, the United States Centers for Disease Control, or CDC, warned that the US was at risk of a severe flu season, since the mild flu season last winter may have led to reduced immunity in the general population.

"This low flu activity was likely due to the widespread implementation of COVID-19 preventive measures, like masks, physical distancing, and staying home," CDC Director Rochelle Walensky said in a briefing on flu vaccination efforts.

"I get it, we are all tired of talking about vaccines," Walensky added, before noting it had become "doubly important" to seek treatment as "we are preparing for the return of the flu".

The UK government launched a new television commercial last week urging people who are eligible for free flu vaccines and COVID-19 boosters to book their shots ahead of the colder months.

The government said the 2021/22 flu campaign will be the "biggest in history" and that it anticipates the flu will pose a far larger threat this year.

"Not many people got flu last year because of COVID-19 restrictions, so there isn't as much natural immunity in our communities as usual," said England's deputy chief medical officer, Jonathan Van-Tam. "We will see flu circulating this winter; it might be higher than usual and that makes it a significant public health concern."