Researchers: Flu and COVID-19 ‘twindemic’ is possible this winter

Sean D. Hamill
Pittsburgh Post-Gazette

PITTSBURGH — Using two separate models, two new studies released Tuesday by University of Pittsburgh researchers found that without an increase in flu vaccination rates this coming winter, the United States could be at risk of a “twindemic” with both seasonal flu and COVID-19 cases at high levels.

That possibility is due to the “rather dramatic decrease in influenza last (2020-21) season that occurred because of the COVID-19 mitigation strategies — closing schools, wearing masks, social distancing,” said Dr. Mark Roberts, senior author on both studies and director of the Public Health Dynamics Laboratory at Pitt’s Graduate School of Public Health. “The decreasing amount of influenza that existed last season has the potential to have a rebound this season.”

Combine that with another possible surge in COVID-19 cases this year similar to what occurred last winter as people spent more time indoors during the colder months, “this is a real problem that some people call the potential this winter for the twindemic,” he said during an online news conference Tuesday.

“We have hundreds of thousands of people getting COVID every day, ICUs and hospitals in the South and in parts of the Midwest are stretched near the breaking point because of COVID-19,” he said. “And as the winter progresses, we normally have hundreds of thousands of people who get admitted to the hospitals with influenza, and if that happens on top of the current COVID pandemic there is the tremendous chance that some of these hospitals will be stretched beyond the breaking point.”

Studies: The two studies Roberts oversaw, both funded by the federal Centers for Disease Control and Prevention but not yet peer reviewed, found similar figures in the most common scenario of an increase of about 20% in flu cases and hospitalizations across the country this coming winter. If 20% more people were hospitalized with flu this winter over the typical 500,000 people, that would mean another 100,000 hospitalizations just from flu.

One of the two studies used a mathematical projection based on factors from the last eight flu seasons, while the other “agent-based” study tried to model every person’s circumstances in Allegheny County as an example.

“Two completely separate modeling methodologies gave surprisingly similar results,” Roberts said, “and that always gives you confidence you’re probably on to something that makes sense.”

Both studies also found that if that does occur, the best way to prevent a surge in influenza cases is to increase flu vaccinations by 20% to 50% this season over past seasons, when typically about half of all Americans get the flu vaccine.

The fear that flu cases and hospitalizations will jump dramatically this winter is because “it turns out that much of the immunity (from contracting flu) that we have as a population occurs because people in the population had influenza last year and if we get a similar (flu) strain circulating they won’t get influenza the next year.”

The result of that, he said, is “the potential to dramatically increase the number of flu cases this coming season” since our overall immunity to flu — the combination of people who get vaccinated this season plus those who became infected last season — will be lower this year.

As an example of how sparse flu cases were last season thanks to the COVID-19 mitigation efforts everyone experienced, Allegheny County in the 2020-21 flu season had just 328 lab-confirmed flu cases, compared to 13,889 in the 2019-21 flu season.

Even though the flu vaccine, which is 40% to 50% effective every year because the flu virus has many different strains that change annually, is “nowhere near as effective as the COVID vaccine” — which is more than 90% effective — Roberts said the better we will all be with a flu shot.

“But, still, the flu vaccine is a tremendous improvement over having no protection at all,” he said. “The more people we can vaccinate, and continue to do mitigation strategies, will help reduce the size of the influenza impact.”

Unprecedented: He said that if we do end up with the feared “twindemic,” it would be historically unprecedented.

“I do not know of another similar situation to what we’re seeing right now,” he said of the possibility of a surge in two respiratory diseases like this.

Of particular concern, he said, is the impact a surge in flu cases could have on children, who, like adults, get protection from previously having been infected, but who were largely shielded from flu and other viruses last year because of the COVID-19 mitigation efforts.

“There is hope, however,” he said. “Both papers demonstrate that increased vaccination works to mitigate this problem. And it’s not only important in adults but in children, who are a major transmitter of influenza.”

As for southwestern Pennsylvania, which has a relatively high vaccination rate for COVID-19 and has not had hospitals stretched to the limit like in the South, Roberts was optimistic.