The flu shot is as familiar an October ritual as football, foliage and Halloween.
But health officials are urging Americans to get the new flu shot and COVID booster at the same time -- the sooner the better.
"Right where we are now -- that's a good time to be vaccinated," influenza expert Dr. Lisa Grohskopf of the U.S. Centers for Disease Control and Prevention told the nation's physicians in a conference call late last week.
That's ahead of time, by traditional measures. Flu season most often peaks in February -- and our levels of protective antibodies are at their highest about two weeks after the shot, then wane over the next four to six months.
Yet this year's season could start early if it follows the pattern seen elsewhere in the world. So a delay could catch people unprepared.
There's another concern: People may not want to make two trips to the vaccination clinic -- so they may get the new COVID booster but fail to return for the flu.
Is there a perfect time to be vaccinated?
If you have a crystal ball, "it's 14 days before the flu attacks the community that you're living in," said Dr. Darvin Scott Smith, clinical lead for flu vaccination at Kaiser Permanente Northern California, who has already gotten his shot.
Here's the problem: Nobody knows when that will be.
Nearly four decades of CDC data shows that 45% of flu seasons peak in February.
But 18% of the time, the season peaks as early as December. In another 16%, it peaks as late as March. Protection isn't assured until two weeks after your shot.
"It is impossible to predict the flu season with any accuracy," said Dr. Kelly L. Moore, president of Immunize.org, a nonprofit group that works to increase immunization rates.
If you want to save time and travel, said Moore, get your flu shot when you get the new COVID booster, now widely available at California's pharmacies and clinics. It's safe and will spare you a return trip. There's no data to show that side effects will be worse.
A flu shot won't protect against COVID, and a COVID shot won't protect against flu. The two vaccines are very different.
"I really believe this is why God gave us two arms -- one for the flu shot and the other one for the COVID shot," White House COVID coordinator Dr. Ashish Jha said at a Sept. 6 news briefing.
Children who need two doses of the flu vaccine -- those six months through 8 years who have never been vaccinated -- should receive their first dose immediately, said experts.
A September shot will create antibodies that can persist long enough to help fend off a later infection, experts said. And even if they don't, you'll get less seriously ill than if you weren't vaccinated at all.
"I'm going to try to get my flu vaccine at the earliest opportunity," said Dr. Bali Pulendran, professor of immunology at Stanford University School of Medicine
"Even if the durability of the antibody response is just a few months, I should be good throughout the season," he said.
September also offers a practical advantage: It's easier to get an appointment. Everyone won't all be rushing in at once, as could happen once the virus arrives.
October is the optimal time from an immunological perspective, experts said. Like all cells, antibodies die of old age. A Kaiser study found a 16% increase in the odds of catching the flu every additional 28 days after peak protection.
That's especially true for older adults, who experience a greater waning of protection than younger people.
"Just don't forget," said Moore. "When the opportunity arises, get it."
If you're not vaccinated by October, it's not too late. Vaccines help as long as flu viruses are circulating.
Once spring comes, you may be worried about protection. But don't get a second flu vaccine, said Smith.
Forecasting a flu season is always a challenge. It can vary in different parts of the country. Every year is different.
Because COVID has changed our behaviors, "the old rules -- what we knew about when flu starts, when it ends -- may not work this year," said UCSC infectious disease expert Dr. Peter Chin-Hong, who aims to get his shot in mid-October.
"I wouldn't game the system," he said. "If the flu has a slow burn, you'll want it before it peaks."
There are three reasons to be cautious, said Smith.
Based on this year's experience in the Southern Hemisphere, flu season could come early. U.S. health officials look to Australian trends for guidance -- and cases there started in April instead of the usual June.
It also was a worse season than the two previous years when people were masked and distancing, said Kaiser's Smith. Behaviors have changed. People are going out more.
Finally, we have less overall immunity to the flu because we've been sitting it out for two years, with lower vaccination rates and reduced exposure to the flu virus.
Last year, flu season was mild but ran long. Experts were surprised by a second small peak, with cases jumping in April and May.
Infectious disease trends "are all whacked out," said Chin-Hong. It's not just flu -- the timing of the common respiratory syncytial virus (RSV), monkeypox and other pathogens have proved startling, he said.
To be sure, flu vaccines are far from perfect. CDC data shows that efficacy ranges widely from year to year, falling to 19% in 2014-15 and climbing to 52% in 2013-14. This year's vaccines are "quadrivalent," meaning they target four different strains of the flu virus; of these, two are different from last year's shot.
Circulating viruses may also genetically drift over time, so a vaccine that is well matched in September may be mismatched in March.
Scientists are now striving to build a better flu vaccine, so it's less critical to time shots perfectly, said Pulendran.
The biggest worry now is not whether the shots are perfectly scheduled -- but that people will skip the vaccines altogether, or just forget, said Moore.
"If you sit down at the Thanksgiving table with someone who is sick," she said, "it's too late."