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After a two-year break, the flu is back. An early rise in cases in Australia has alarmed public health authorities – and should prompt the United States to put the familiar virus back on the public radar.

“There’s no doubt that we’re going to have a big season,” said Ian Barr, deputy director of the World Health Organization’s Collaborating Center for Influenza Reference and Research. Even though testing in Australia has increased, identifying more people with the flu, the number of cases is following a similar trajectory to that seen during the 2019 flu season, a peak year for flu in Australia. Argentina and South Africa are also showing modest early signs of a tougher flu season.

Activity in the southern hemisphere isn’t always a good predictor of what will happen in the northern hemisphere, but signs of returning flu should at least be a warning in the United States, where enthusiasm for the flu vaccine was low last year. As with Covid vaccines, flu shots may not prevent people from catching the virus, but they can prevent the worst consequences of infection.

Vaccine complacency could be dangerous the first fall and first winter without the Covid rules. Public health authorities must ensure that access to vaccines is as easy as possible.

The past two years have been weird for the flu. During the first flu season of the pandemic, cases in the United States were virtually non-existent. Even in the second, activity did not follow the typical pattern. The largest (still small) spike came in April — the first time it was so late, says Lynette Brammer, who leads the Centers for Disease Control’s domestic flu surveillance team. In some parts of the country, last winter’s flu continues to circulate.

“It’s just crazy that we’re sitting here on June 2 and still having significant flu activity,” Scott Hensley, a University of Pennsylvania professor who studies flu, told me.

The most obvious explanation for these quiet and weird flu seasons is that limited international travel, social distancing, mask mandates and other Covid mitigations have given the flu virus little opportunity to spread. The late rise in cases this spring came when mask mandates and other Covid rules were lifted.

A second theory, which has yet to be proven, is that infection with one virus prevents a second virus from taking hold. The dreaded “double epidemic” of both flu and Covid has yet to materialize. But without evidence that viral interference is real, the possibility remains that next winter could bring waves of both viruses.

At this point, a return of the flu could wreak more havoc than usual. After two seasons of low activity, more people lack immunity. Children, usually exposed to the flu at the age of 3, are particularly naïve to the virus.

It is concerning that flu vaccination rates have dropped, including among the most vulnerable groups. Over the past two years, the proportion of children who have had the flu shot has risen from 62% to 55%. Among pregnant women, the vaccination rate fell to 52% from over 65%.

Of course, during the pandemic, many people have seen less of their health care providers, and no doubt many are feeling some vaccine fatigue as well. People who are still working from home have missed flu vaccination campaigns in the office.

But public health authorities need to get vaccination rates back up. Several states and territories in Australia have taken the unprecedented step of offering free shots. Research shows that people are more likely to get vaccinated if recommended by their doctor and offered to them. Businesses should start providing vaccines at work again, pediatricians’ offices should think about how to make sure parents can get vaccinated with their children, and school districts should work to increase vaccination rates. against influenza in children.

At the same time, it looks like another round of Covid boosters will be needed in the fall. People need to know that it’s perfectly normal to get the Covid and flu shots at the same time. And coordination is needed to ensure that places offering one have an adequate supply of the other.

More from Bloomberg Opinion:

• Like flu shots, Covid reminders need an annual schedule: Lisa Jarvis

• What is worse than a pandemic? A Twindemic: Theresa Raphaël

• Do you realize that Covid-19 could return in the fall? : Justin Fox

This column does not necessarily reflect the opinion of the Editorial Board or of Bloomberg LP and its owners.

Lisa Jarvis is a Bloomberg Opinion columnist covering biotech, healthcare, and pharmaceuticals. Previously, she was the editor of Chemical & Engineering News.

More stories like this are available at bloomberg.com/opinion