With flu season approaching, plus the possibility of another surge of Covid cases this fall, many Americans are wondering two things: When is the best time to get the flu vaccine – if there is one – and is it OK to double up and receive both the seasonal flu vaccine and new omicron-specific Covid-19 booster at the same time?

Though the CDC says it's completely fine to get a flu vaccine and Covid-19 shot simultaneously, there hasn't been specific guidance pertaining to the newly formulated booster.

Yet, pharmacies like Walgreens and CVS are offering the vaccines as a packaged deal, and people have already begun getting immunized with both shots at the same time.

But not all public health experts believe this is the best approach, especially given the average flu shot timeline that the U.S. has followed since before the pandemic, says Theodora Hatziioannou, a virologist and associate professor at Rockefeller University.

"For the average person, probably having it a little bit later would be better," Hatziioannou says, "I'm certainly waiting."

'When should I get my flu vaccine?'

The timing of when you get your flu shot should be aligned with the time frame that would allow you to have the best antibody response during the beginning of flu season, Hatziioannou tells CNBC Make It. 

It's important to look at data from recent years, prior to the pandemic, in your local area to determine when cases historically start to increase in your community, Hatziioannou says.

The standard rule of thumb is get your vaccination four weeks before the beginning of flu season in order to have the most protection because that's when you'll have the best antibody response, she notes.

"In New York and several other areas with the same geographical latitude, flu season usually starts in December and lasts until March," she says, "So, if you have it [the flu shot] now, at the end of September or [early] October, your peak antibody responses will be in November. It's a little bit too early."

At this time, Hatziioannou recommends getting your flu shot at the end of October or early November to have the highest protection throughout December.

And the protection provided by the vaccine generally wanes over time, so if you get the vaccine too early, you could have less antibodies at the tail end of flu season, she says.

"The one thing I would say is this year I would definitely get both shots because the combination could be quite nasty," Hatziioannou says.

"You don't want to get the flu while Covid's going around, and you don't want to get SARS-CoV-2 while the flu is going around. I can envision that a double whammy would be really terrible."

'Should I get my omicron booster and flu shot at the same time?'

There aren't any specific reasons why you can't get your omicron booster at the same time as your flu vaccine, says Mark Conroy, emergency medicine physician at the Ohio State University Wexner Medical Center. 

"In terms of side effects from the omicron-specific vaccine and the flu shot, there haven't been any studies specifically looking at those two together," Conroy says, "But there have been some studies looking at the prior booster with the flu shot, and there really wasn't a negative effect related to having both shots at the same time."

And if you're worried about not being able to find availability later in the year to get your flu shot, then Conroy encourages you to get a dual-appointment. 

"If you don't get them at the same time, what's the biggest risk? And the risk is just forgetting to get the flu shot later," he says, "If you have the chance, obviously get vaccinated over not getting vaccinated."

But, if you can wait for your flu shot, he does recommend doing so until mid- to late-October or the beginning of November as Hatziioannou suggested.

"From the standpoint of the omicron booster, my recommendation is people should just get it as soon as they're able. Covid is around enough and making people sick," he says, "September is a little bit early for the flu shot. I generally recommend getting it a little bit later in the fall."

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