Health experts are anxious to prevent a 'catastrophic' winter flu season

LONDON — Mass vaccination campaigns are being rolled out across the developed world, but many countries are still contending with surges in coronavirus infections and new strains, such as the highly infectious delta variant.

And now health experts are warning the public that there could be a very difficult flu season ahead too.

"There is a lot of uncertainty about the 2021-2022 influenza season," epidemiologist Lauren Ancel Meyers, director of the University of Texas Covid-19 Modeling Consortium, told CNBC.

"As with Covid, when somebody recovers from a seasonal influenza infection, they retain some level of immunity that protects them against future infection, at least for a short period of time. Since our Covid mitigation measures prevented influenza transmission last year, there are not a whole lot of people who were recently infected," she said.

"So we may be entering flu season with a higher level of susceptibility than usual, which could exacerbate the risks," she added.

Whether or not the flu season turns out to be more severe this year could depend both on the evolution of the virus and decisions made at a personal level, Ancel Meyers believes.

"As we have learned from the last 18 months of the Covid-19 pandemic, the decisions we make as individuals and communities can have a huge impact on the fate of an outbreak. We can and should do our part to prevent a catastrophic flu season, by getting vaccinated early this fall and taking sensible precautions if and when the virus starts spreading widely," she said.

"Our experience with Covid may lead to behavior changes that work in our favor. People may be more willing to take influenza vaccines and to wear face masks or take other precautions to prevent transmission during the peak of the season."

The alarm over a potentially bad winter flu season was raised by Professor Chris Whitty, England's chief medical officer, back in June.

"Either we will have a very significant Covid surge, people will minimize their contacts and we will have less respiratory viruses, or people will be back to a more normal life, there will be some Covid but on top of that we will go back to having a flu surge, an RSV (respiratory syncytial virus, a common respiratory virus that usually causes mild, cold-like symptoms) surge in children, and so on."

"I think we need to be aware of and brace for the fact that the coming winter may well be quite a difficult one," he said.

Flu figures from the U.S. and England show that flu illnesses dropped during the pandemic largely due to the social-distancing measures in place helping to stop transmission. During the 2019-2020 flu season, the U.S. Centers for Disease Control and Prevention predicted that influenza and pneumonia (a life-threatening complication of flu which often affects older people) was associated with 38 million illnesses, 405,000 hospitalizations, and 22,000 deaths. The CDC stressed that the figures were just estimates.

But when it comes to the 2020-2021 season, the CDC told CNBC that because influenza activity was low last winter "there were not enough flu illnesses or flu-associated hospitalizations in the United States to use a model to estimate the U.S. flu burden for 2020-2021."

"We can say that the low level of flu activity during 2020-2021 season contributed to dramatically fewer flu illnesses, hospitalizations, and deaths compared with previous flu seasons," Lynnette Brammer, team lead in the CDC's Domestic Influenza Surveillance Team told CNBC Tuesday.

"For example, during the three seasons before the pandemic, the peak proportion of respiratory viruses testing positive for flu on a weekly basis was between 26.2% and 30.3%. Last season, however, the proportion of respiratory viruses testing positive for flu remained lower than 0.4% during every week of a typical influenza season."

In England and Wales, for comparison, deaths caused by flu and pneumonia in 2018 totaled 29,516 in England and Wales and amounted to 26,398 in 2019, according to figures from the Office for National Statistics. Similarly to the U.S. there was a sharp drop in 2020, with 15,437 deaths involving (and due to) influenza and pneumonia.

Whitty's comments were echoed by Neil Ferguson, an epidemiologist at Imperial College London who has also advised the British government on its Covid strategy.

He agreed that "seasonal influenza is likely to be a significant issue" coming into the fall and winter.

"All the measures we adopted against Covid around the world drove flu to very low levels and basically nobody got infected with flu last year and so immunity has dropped a little ... I think we do need to be prepared for a potentially quite significant flu epidemic later this year," he told the BBC's "Today" show in late June.

It's difficult to predict what will happen during the 2021-22 flu season, the CDC's Brammer said, but the CDC "is preparing for flu virus circulation to return to pre-pandemic levels" given that the circulation of some respiratory viruses is already returning to pre-pandemic levels.

"We anticipate that something similar may happen with flu, especially as community mitigation efforts continue to be relaxed. Data from the National Respiratory and Enteric Virus Surveillance System (NREVSS) show that activity for some common respiratory viruses, including respiratory syncytial virus (RSV), which also circulated at low levels during the 2020–2021 season, is increasing. This increase is outside of its typical season," she noted.

Several factors "could lead to the upcoming flu season being more severe than usual," Brammer stated:

"We know that flu vaccination remains the best way to protect yourself and your loved ones against flu and its potentially serious complications," Brammer added.