You know you should get your COVID and flu vaccines. But when?

You can’t miss all the reminders to get your flu and COVID-19 shots this fall. They’re everywhere: in health emails at work, social media advertisements, pharmacy window displays. As for when to get those vaccinations, though, the reminders don’t offer much help.

Even as vaccine experts acknowledge that every person’s situation is different, there is some consensus on timing that applies to most of us. Waiting on a flu shot until mid-to-late October, experts say, can help maximize protection through the peak infection season. On the other hand, there is no particular reason for most of us to wait for a COVID-19 vaccine now that the updated ones are available. 

What matters most is to make sure you get both vaccinations, according to the U.S. Centers for Disease Control and Prevention (CDC), which recommends everyone over six months old get both newly updated vaccines this fall.

Still, timing does matter, especially for a highly seasonal virus like the flu, which is most prevalent in the United States from December to February.

“The chance of me getting exposed to influenza in the next month or two are very, very low, so I’m going to wait,” David Topham, an immunologist at the University of Rochester who studies flu vaccines, said in mid-September. “I want the most immunity when the viruses are going to be most prevalent, so I like to get vaccinated in late October.”

The immune response generated by a flu vaccine lasts about six months, which is why experts advise against most people getting vaccinated in August or early fall. Dr. Jeremy Faust, an emergency physician at Brigham and Women’s Hospital in Boston, points to the waning protection from vaccines as the reason he gets vaccinated in late October in a recent guest essay for The New York Times. 

A vaccination any time is still better than none, said Paul Thomas, an influenza specialist at the St. Jude Children’s Research Hospital in Tennessee. “If it’s convenient for you to get it at a particular time, then just get it. It doesn’t hurt you to have gotten it a bit early, and that’s more important than not.” 

The CDC starts pushing vaccination in September to give more time for people to get vaccinated before flu season starts. The CDC focuses on overall vaccination rates instead of maximizing the chance of avoiding severe illness, Faust said. Studies have shown that the benefits of delayed vaccination can be negated if fewer people get vaccinated, so the bottom line is always that any vaccine is better than no vaccine.

While timing for the flu is relatively predictable, the COVID-19 virus is not as obviously seasonal. COVID-19 is circulating widely in the United States now, and there is no specific reason to wait to get vaccinated, Topham said. With the COVID-19 vaccine, everyone’s individual risk is a bit different — maybe you have travel plans or family gatherings later this year that could expose you, for example.

“We’re having a vaccine clinic here next week where I can get a COVID vaccine. And I might go get my COVID vaccine because everybody’s coming back to school, we are inside a lot more and my chances of getting exposed may be higher,” Topham told Scienceline in mid-September.

The COVID-19 and flu shots work differently, but both have the same goal: to update and fine-tune your immune response. This is a continual process for viruses like the flu and COVID-19 because the viruses themselves change over time.  

Viruses are opportunistic: They find weak spots in a population’s immunity, and the strains that slip through the defenses survive and continue spreading. This leads to a gradual evolution in virus make-up, and the need for updated vaccines.

Getting a vaccine both decreases the chance of infection and raises your baseline immunity, helping prevent serious illness, Topham said. It is important to remember that they take a few weeks to become fully effective, then start to lose their effectiveness after a few months. 

With a flu shot, it takes about two-to-four weeks to gain maximum immunity, because your immune cells have to process the proteins from the vaccine and then instruct your immune system to make antibodies against those proteins, Thomas explained. The protection the vaccine provides declines by a little less than 10% a month, research suggests.

For COVID-19, the most common vaccine in the United States is an mRNA vaccine, which works differently than the typical flu vaccine. With mRNA vaccines, there is nothing for the immune system to respond to right away. Instead, the mRNA has to get into a cell and be translated into a protein that the immune system can recognize. That process can take about two weeks, according to Johns Hopkins Medicine.

Much like the flu, the updated COVID-19 vaccine offers the best protection for the first few months after vaccination. When to get vaccinated, however, depends largely on personal risk factors.

“If I had any underlying health conditions, or if I lived with somebody who had underlying health conditions, … I might get vaccinated sooner, because I’m not only protecting me, I’m protecting them,” Topham said. 

A recent infection might also change the timing; the CDC said someone who recently had COVID-19 can delay getting an updated vaccine for three months. 

As for getting both vaccinations at the same time, there is no significant immunological downside if that is what is most convenient for you, according to Topham. You may notice stronger side effects, he added, but they should subside after a day or two. 

“The vaccines are not perfect,” Topham said, “but we know at a population level, it’s much better for everybody if we get as many people vaccinated as possible.”