Early in the pandemic, an exposure to COVID-19 meant waiting anxiously for many days to see if you were infected. Now, the window is getting smaller and smaller.
In a 2022 review published in the journal JAMA Network Open, researchers analyzed 141 studies to determine how COVID-19’s incubation period ― the time from when you get infected to when you start showing symptoms ― has changed since March 2020.
The review, which was conducted by scientists in Beijing, found that with every new variant, COVID-19’s incubation time has decreased significantly. Omicron had the shortest time between infection and symptoms. (EG.5, the subvariant that is currently the most dominant COVID-19 strain in the U.S., is a descendant of omicron.)
“The incubation periods of COVID-19 caused by the Alpha, Beta, Delta, and Omicron variants were 5.00, 4.50, 4.41, and 3.42 days, respectively,” the 2022 review stated.
It’s worth noting that the studies analyzed in the review largely relied on people recalling their date of infection and the date symptoms started from memory, so there is room for error if study participants misremembered.
Additionally, an April 2023 study concluded that incubation times for omicron variants were shorter than that of delta variants. Specifically, the incubation time for BA.1, which was the initial omicron subvariant, was three days while the incubation time was four days for delta.
In new reporting, experts told HuffPost that they still agree with these findings and are seeing a short incubation period in their own work, too.
“While the incubation period for COVID-19 can be anywhere from 2-14 days after exposure to the virus, with the omicron variants currently circulating we see incubation periods much shorter,” David Souleles, the campus public health response team director at the University of California, Irvine, told HuffPost via email.
Now, people show symptoms two to five days after exposure, according to Souleles. And, as research has found, “with pre-omicron variants, the [incubation] periods tended to be longer,” he said.
A shorter incubation period means COVID-19 can spread more easily.
In previous reporting, Souleles told HuffPost that when you become symptomatic faster, you have more of the virus circulating in your system. This makes you more likely to spread the sickness to someone else.
“Symptoms, so coughing and sneezing, accelerate the ability of the virus to move from person to person,” he said.
Plus, since there is no longer a window of seven to 10 days between infection and symptoms, the virus doesn’t have to hang out in a person for a extended period of days before infecting someone else, Souleles explained. With earlier variants, there was a longer gap of time when the virus was building up in the system but not able to be transmitted to another person, he said. Now, that isn’t the case.
Say, for example, that you had dinner with a friend on Saturday who informs you on Sunday that they just tested positive for COVID-19, and you may have been exposed at the dinner. If you were infected, chances are your symptoms will appear pretty fast, like by Tuesday or Wednesday.
There’s also a possibility that you may have spread the virus to other people within that short time frame, which was less likely with incubation windows for variants like alpha and beta. With those earlier variants, the virus required more time to build up before being transmissible.
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The shorter incubation period achieves the virus’s goal: to infect more people.
The 2022 JAMA review stated that since the start of the pandemic, COVID-19 “has evolved and mutated continuously, producing variants with different transmissibility.”
This is because the virus develops changes to its structure that make it more invasive, so “it can do what viruses want to do — continue to infect people,” Dr. Gregory Poland, a professor of medicine and infectious diseases at Mayo Clinic, previously told HuffPost. The shorter incubation period is one of the ways the virus has mutated to become more contagious, Poland explained.
COVID-19 is now hyper-contagious and has “developed the capacity to infect the upper airway more than the lower airway,” which makes it that much more transmissible, too. If a virus is in your upper airway (your nose and your throat), it’ll have an easier time spreading through the respiratory route compared with a virus that is in your lower airway (like the lungs).
How much more contagious are current strains? Poland explained that “if [omicron] had shown up and not the original strain [in 2020], we wouldn’t be talking about 1 out of 308 Americans being dead, we would probably be talking about 1 out of 200.”
As time goes on and COVID-19 continues to infect people, the virus mutates to become more contagious, resulting in new strains.
To protect yourself, get the updated COVID-19 shot in the fall.
This all may seem pretty grim, and understandably so. But there are ways you can keep yourself and your loved ones safe.
“The one thing people can continue to do is make sure they are up to date with their vaccinations,” Souleles said.
Updated COVID-19 shots for 2023 have been approved by the Centers for Disease Control and Prevention and are expected to be available any day. The new shot will target the XBB.1.5 subvariant ― another omicron descendant that was dominant for much of the summer and remains a prevalent strain ― although EG.5 is now dominant in the U.S. Still, the boosters are expected to offer strong protection against currently circulating omicron variants as a whole, including XBB.1.5 and EG.5.
“When the updated dose becomes available, be sure to get vaccinated,” Souleles said. “While you are at it, get your seasonal flu vaccine too. Both can be provided at the same time.”
And continue to follow the mitigation measures we know well.
You’ve heard this hundreds of times in the past three-plus years, but it’s important to follow the rules that keep COVID-19 from spreading ― especially as it’s more contagious than ever.
Wash your hands for 20 seconds, wear a mask indoors, opt for outdoor settings when possible, stay up to date with your vaccinations, take a COVID-19 test if you feel sick, and isolate if you test positive.
A previous version of this story appeared in August 2022.
Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.
