COVID and the flu: Is a ‘twindemic’ threat lurking again?

A Fluzone influenza vaccine at Pharmaca Integrative Pharmacy in San Francisco on Jan. 9, 2018. AP Photo/Jeff Chiu

First, the good news: The flu was practically nonexistent last year.

The bad news: Little flu last season means increased risk this fall and winter.

The so-called “twindemic” that public health officials in California and elsewhere warned about last year — the combined threat of influenza and COVID-19 — was largely eased by the wide use of face masks, physical distancing and reduced travel, experts say. But their concerns are back this year.

Flu activity during the 2020-21 season was the lowest recorded since data began being collected in 1997, according to the Centers for Disease Control and Prevention.

“Because of so little disease last year, population immunity is likely lower, putting us all at increased risk of disease this year, especially among the most vulnerable including our children,” CDC director Rochelle Walensky said at a press briefing earlier this month. 

The agency was notified of one pediatric death last flu season, compared to 199 flu deaths reported among children the year before. The CDC did not have estimates for last season’s flu deaths among adults.

A potentially bad flu season is especially concerning as hospitals are facing severe understaffing. Last week, a third of all California hospitals reported critical shortages in staffing due in part to burnout, CalMatters reported.

For example, during the 2017-18 flu season, considered severe by the CDC, about 710,000 people were hospitalized nationwide — including about 100,000 people in California, health officials have estimated. 


Flu shots continue to be the best protection against severe illness. Doctors and public health experts are urging people to get vaccinated — ideally by the end of October — noting that flu shots will ultimately reduce the number of people who will need ambulance transport and the emergency room.

“We saw virtually no flu last year and are uncertain about what this year will hold,” said Dr. Penny Borenstein, San Luis Obispo County’s health officer. 

“We do know that the most recent flu season was relatively mild in the Southern hemisphere and that the vaccine was a good match for the strains circulating there,” she said. “We are hoping for a similar outcome during our soon-to-arrive Northern hemisphere flu season.”

According to CDC estimates, 47% of adults in California received a flu shot last year, similar to the 2019-20 flu season — that’s among the lowest in the country and slightly lower than the national average of 50%. By comparison, Massachusetts and Rhode Island had the highest flu shot uptake last year at 62% and 64%, respectively. 

California fared better with flu shots for kids — about 58% of children were immunized last season. In Massachusetts, immunization among kids was close to 84%. 

Last year, many people skipped holiday gatherings, college students weren’t traveling home, and masking and physical distancing was required in most places — all factors that probably helped mitigate flu activity. 

“Unfortunately, this year we have relaxed most of these measures,” said Shira Shafir, an epidemiology professor at the University of California, Los Angeles. That’s bad news for both COVID and non-COVID respiratory infections, she said. 

And the pandemic isn’t quite over. While close to 80% of Californians age 12 and over have been immunized against COVID, that still leaves pockets of people who can flood hospitals this winter.

“It can still get bad,” said Dr. George Rutherford, a professor of epidemiology at University of California, San Francisco. “I do think all of this will be revealed in November — if we start to see a surge in early November, it will get worse during the holidays.”

Health experts are also keeping an eye on the delayed uptick of respiratory syncytial virus, or RSV, another seasonal virus. This virus usually peaks in the winter, but in some places, cases spiked this summer, much later than usual, Rutherford said.

Photo illustration by Rayne Zaayman-Gallant of the European Molecular Biology Laboratory (Creative Commons CC BY-NC-ND 4.0)

This virus can result in common cold symptoms or even pneumonia. People of any age can become infected with RSV, but babies and older adults, especially those with compromised immune systems, are at increased danger of severe cases

Like the flu, respiratory syncytial virus diminished last winter likely because of distancing and masking. Surveillance reports from the CDC show that, in California, the percentage of tests that come back positive for RSV has been on the rise since June.

“That’s also a concern, maybe even more than influenza,” Rutherford said. 

Researchers who looked at the most recent RSV spike in New York recently wrote in the Journal of American of Pediatrics that hospitals should be planning for an increase in pediatric emergency visits. There is no vaccine for this virus.

The burden respiratory illnesses will have on this fall and winter depend largely on how much masking continues, Rutherford said. Besides getting the flu shot and the COVID-19 vaccine, he recommends that people plan to celebrate Thanksgiving outdoors.

“Also, you don’t need to have 60 people over for Thanksgiving. Keep it small, make life simpler,” he said.

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