Although the U.S. had one of its mildest flu seasons in 2020-2021, flu vaccine distributors and manufacturers were not negatively affected during that period. Their sales were locked in before anyone knew there would be a global Covid-19 pandemic.

The companies have noted softer sales so far this year, however. And because of the way their distribution systems are set up, there might be fewer shots available for the upcoming flu season. That, coupled with relaxed mask wearing and increased travel, could lead to bad outcomes once the season hits.

At the start of last year, many distributors had a bumper “prebooking” period, in which orders were placed for the upcoming season, said Michael Einhorn, president of Dealmed, a Brooklyn-based distributor of medical supplies and vaccines. Distributors prebook partly based on their sales the season before, then manufacturers produce a specific number of doses, he said.

Seqirus, a vaccine manufacturer in Summit, N.J., increased its flu shots output by 20% for the 2020–2021 season, said Dave Ross, vice president of North American commercial operations.

Flu season typically runs from October to May, according to the federal Centers for Disease Control and Prevention. The medical community had been expecting the 2020–2021 flu season to be worse than the one before it, Ross said. But according to CDC, the hospitalization rate this flu season in America was just 0.7 per 100,000 people—the lowest since the agency started tracking flu data in 2005.

Covid-related measures such as masks and social distancing brought the current flu rate to a new low. Another factor is that the pandemic might have prompted more residents to seek out a flu shot.

“We were sold out of our flu vaccines by October, and there was no way to get any more,” Einhorn said. “Perhaps because the Covid vaccine was not available yet, people were eager for any way not to fall sick, and covering their bases with flu was one such way.”

In October New York City reported a 37% increase in adult flu vaccinations and a 27% boost for children, compared with the previous season.

Lower flu rates have in turn dampened expected vaccine demand for the 2021–2022 flu season. With demand lower, the amount of doses being manufactured is likely lower too.

A spokeswoman for Henry Schein, a medical supplier in Melville, Suffolk County, pointed to the company’s first-quarter earnings, which show muted sales. Excluding sales of personal protective equipment and other Covid-related products, the distributor’s medical sales were down 7%, according to its financial filings.

In September Dealmed was expecting a 25% increase in this year’s prebooking, but there has been uncertainty from providers about how much to order for the upcoming season, Einhhorn said.

“We initially saw strong early numbers,” he said, “but then we suddenly saw sales fall off.”

He added that acquisition costs for the vaccines were up 20% on average this year, placing a further squeeze on that segment.

Some stakeholders are concerned the next flu season might be particularly nasty.

There could be a surge in the winter, especially in light of the reopenings occurring now, warned Dr. Purvi Parikh, an allergist and immunologist at NYU Langone Health.

“Due to this current flu season being so mild, people might have their guards down and might be less aggressive about getting their flu shot,” Parikh said. People are gathering and traveling more, she noted.

Even if the next flu season is a bad one, there are reasons to be optimistic, said Dr. Waleed Javaid, director of infection prevention and control at Mount Sinai Downtown.

“Flu cases went drastically down not solely because of vaccines but because of masking and social distancing,” Javaid said. With people more cognizant of the benefits of masking, especially when they’re sick, the practice could become more common, mitigating the severity of the flu season, he said.

But, he said, it’s important to start talking about the risks now.

“If we don’t plan early,” he said, “that worry of a bad season will come true.”

This story first appeared in our sister publication, Crain’s New York Business.