Early in the pandemic, an exposure to Covid meant waiting anxiously for many days to see if you were infected. Now, the window is getting smaller and smaller, according to a new review published in the journal JAMA Network Open.
Researchers analysed 141 studies to determine how Covid’s incubation period – the time from when you get infected to when you start showing symptoms – has changed since March 2020. The study, which was conducted by scientists in Beijing, found that with every new variant, Covid’s incubation time has decreased significantly. Omicron, which is the current dominant variant in the US and UK has the shortest time between infection and symptoms.
“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 study stated.
It’s worth noting that the studies analysed 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 any study participants misremembered.
But, two experts told HuffPost that they agree with the findings and are seeing a shortened incubation period in their own work, too.
A shorter incubation period means Covid can spread more easily.
According to David Souleles, the campus public health response team director at the University of California, Irvine, 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 seven to 10 day window between infection and symptoms, the virus is building up faster and doesn’t have to hang out in a person for a period of days before infecting someone else, Souleles explained. In earlier variants, there was a longer gap of time when the virus was building up in the system but not able to transmit to another person, he said. Now, that isn’t the case.
So, say for example you had dinner with a friend on Saturday who informs you on Sunday that they just tested positive for Covid and you may have been exposed at dinner. If you were infected, chances are your symptoms will appear pretty fast, like by Tuesday or Wednesday.
There’s also a chance you may have spread the virus to other people within that short timeframe as well, which was less likely than in other incubation windows for variants like Alpha and Beta. For those earlier variants, the virus required more time to build up before being transmissible.
The shorter incubation period achieves the virus’s goal: to infect more people.
The study stated that “since the beginning of the Covid-19 [pandemic], [Covid] 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,” said Dr. Gregory Poland, a professor of medicine and infectious diseases at the Mayo Clinic. The shorter incubation period is one of the ways the virus has mutated to become more contagious, Poland explained.
Covid 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 when compared to a virus that is in your lower airway (like the lungs).
Just how much more contagious are these 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 continues to infect people, the virus mutates to become more contagious, resulting in new strains of the virus.
To protect yourself, get the updated Covid vaccine in the autumn
This all may feel pretty grim, and rightfully 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.
He added that it seems many people will have that opportunity to get a booster dose in the autumn that will better target BA.4 and BA.5. Both Pfizer and Moderna will have updated Covid vaccines that provide stronger protection against Omicron than the current iteration of the vaccine.
And continue to follow the mitigation measures we know well
You’ve heard this hundreds of times in the past two-plus years, but it’s important to follow the rules we know keep Covid 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 test if you feel sick and isolate if you test positive.
Experts are still learning about Covid-19. The information in this story is what was known or available at the time of publication, but guidance could change as scientists discover more about the virus. To keep up to date with health advice and cases in your area, visit gov.uk/coronavirus and nhs.uk.