Pitt experts anticipate major flu surge this season, urge flu shots

This year’s flu season may be severe with up to half a million more flu hospitalizations than usual, according to new studies from University of Pittsburgh researchers.

Two studies released Tuesday — both led by researchers at the University of Pittsburgh Graduate School of Public Health — indicated that waning population-level immunity after last year’s historically mild flu season could cause a sharp spike in influenza cases this year.

The studies predict the upcoming flu season could hit young children particularly hard.

“As covid-19 mitigation measures — such as masking, distancing and school closures — are relaxed around the world, we’re seeing a fierce resurgence of other respiratory viruses, which does not bode well for the coming flu season,” said Dr. Mark Roberts, director of the Public Health Dynamics Laboratory at Pitt Public Health and senior author of both studies. “In a worst-case situation with a highly transmissible flu strain dominating and low influenza vaccination uptake, our predictive models indicate the potential for up to nearly half a million more flu hospitalizations this winter, compared to a normal flu season.”

Because last year’s flu season was historically mild, fewer people than usual gained natural immunity from influenza. That could fuel a resurgence of flu cases this year, Roberts said.

Much of the population’s immunity for influenza comes because people contracted the flu in the prior season. Immunity conferred from actually having the virus lasts longer than immunity from the vaccine, Roberts said, though it only protects individuals from strains similar to the one they contracted.

“That pool of immunity that occurs because of the presence of having had disease is not there this year, and so there’s substantially less immunity in the population,” he said.

Children, who do generate better immunity from the flu shot than adults, also get “most of their protection from having had influenza,” Roberts said. Because the number of children who had the disease “has fallen off dramatically” and the number of kids getting the vaccine seems to be “about the same,” Roberts said, “that means in total there is much less immunity existing in kids under 5.”

Using a simulation platform called the Framework for Reconstructing Epidemiologic Dynamics (FRED), Pitt Public Health research assistant professor Mary Krauland led a team that discovered the 2021-22 flu season could have 20% more flu cases than a normal year. Their research suggested there is a potential for the influenza caseload to double.

The pair of studies funded by the Centers for Disease Control and Prevention — which used different modeling platforms, but produced consistent results — suggested that this potential surge of flu complications could be mitigated if vaccination rates jump. Vaccination uptake would need to be between 20% and 50% higher than during recent flu seasons to diminish the potential surge of influenza, researchers said.

During the 2019-20 flu season, about half of Americans were vaccinated against the flu. The platform demonstrated that increasing the vaccination rate by even 10% resulted in a predicted decrease in hospitalizations — though the percent of decrease could be anywhere from 6% to 46%, depending on how transmissible the dominant flu virus is this season.

“Vaccinating as many people against flu as possible will be key to avoiding this scenario,” Roberts said, explaining that mitigation measures used to curb the spread of covid-19 can also help slow influenza outbreaks.

Another Pitt researcher, Pitt Public Health postdoctoral associate Kyueun Lee, led a team who used a Susceptible-Exposed-Infected-Recovered model, which indicated getting about 75% of Americans vaccinated against the flu would plummet additional hospitalizations. The model suggested that this year’s flu season could bring 600,000 hospitalizations — at least 100,000 more than in a typical year.

Vaccine uptake last year reached a record high in the United States, with 193.8 million doses of the flu vaccine distributed nationwide, surpassing the previous record of 174.5 million doses in the prior season.

Health experts last year encouraged high vaccination rates as a way to keep flu cases low as covid-19 threatened to overwhelm hospitals. Doctors are again imploring the public to get flu shots as the highly-transmissible delta variant fuels another surge in covid-19 cases just before the start of flu season.

Last year, a so-called “twindemic” — a simultaneous outbreak of influenza and covid-19 — was avoided, thanks to high vaccine uptake and mitigation measures meant to curb the spread of covid-19. This year, those fears of a “twindemic” have resurfaced.

“If anything, our models show that we should be more concerned this year about the possibility of a surge in covid-19 hitting at the same time as a massive flu outbreak in areas of the country with low vaccination rates against both diseases,” Roberts said.

In parts of the country where covid-19 vaccination rates are low and delta has sparked new covid-19 outbreaks, “there is a tremendous chance that some hospitals will be stretched beyond their breaking point” if flu season is severe, Roberts said.

“My own personal feeling is that we probably are going to be OK here in Western Pennsylvania,” he said, noting that both UPMC and Allegheny Health Network “have all of the modern treatment facilities and have been doing this for a long time” without being “stretched to the breaking point.”

In Lee’s research, models also predicted that, if covid-19 precautions would persist and this year’s flu season would again be blunted, natural immunity against the flu would continue to decline. That would “further increase the odds of an ever-larger flu outbreak with high hospitalization rates whenever social activity returns to normal,” the University of Pittsburgh said.

“This is not to suggest that we should stop covid-19 mitigation efforts to avoid a severe flu season,” said Roberts, who is a professor of health policy and management at Pitt Public Health. “Instead, it shows that more of us — particularly young children — will be susceptible to the flu and that vaccination is absolutely essential to avoiding bad outcomes.”

Flu vaccines are recommended for nearly everyone 6 months and older and Roberts said he anticipated there would be an ample supply of the shots.