It appears to be shorter than the incubation period of other COVID-19 variants, which is important when it comes to testing and transmission.
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The Centers for Disease Control and Prevention (CDC) identified on Tuesday that Omicron—the newest variant of COVID-19—is the dominant strain in the US, accounting for about 58.6% of new cases as of December 25. And as the newest iteration of COVID continues to spread across the US, researchers have been learning more about how it differs from previous strains of SARS-Cov-2. The most recent information: Along with its increased transmissibility and potentially milder symptoms, Omicron may also have a shorter incubation period than Delta and other strains.

According to new data, also released on Tuesday, the CDC suggests that it may take only three days for people infected with the Omicron variant to exhibit symptoms. The study, published for early release for the CDC's Morbidity and Mortality Weekly Report (MMWR), investigated six cases of suspected Omicron infections, all within the same household. According to the CDC, officials suspected the Omicron strain, since the index patient (a 48-year-old man) had recently returned from Nigeria.

Following reverse transcription–polymerase chain reaction (RT-PCR) testing and genomic sequencing of the samples, each family member was found to be infected by the Omicron variant of COVID-19. (It should be noted that only one member in the household was fully vaccinated; the rest were unvaccinated and previously had COVID-19 or symptoms associated with the illness.)

Through further investigation, the CDC and Nebraska Health Department determined the median incubation period—or the median time between exposure and symptom onset—was just 73 hours, or about three days. The CDC noted that this is shorter than the approximately four-day incubation period for the Delta variant, and an approximately five-day incubation period for other iterations of COVID-19.

The CDC data isn't the only information that suggests a shorter incubation period for Omicron: According to a study from the Norwegian Institute of Public Health (NIPH), attendees of a Christmas party held in Oslo also developed symptoms of COVID-19 within three days of being exposed to the Omicron variant. Of 111 attendees at the Christmas party—most of whom were fully vaccinated—73% were subsequently diagnosed with COVID-19, with the assumption that "that most people who [were] ill [were] infected with the omicron variant," according to a press release. "Even though most of the cases have not had a severe disease course at this time, almost all developed symptoms relatively quickly after the Christmas party," the release added, noting the virus' incubation period.

While we've all been inundated with more medical terminology than we'd like to know throughout the course of the pandemic, an "incubation period" may be a new one that many people have been hearing a bit more lately. Here, an epidemiologist—along with other expert sources—help explain what a virus' incubation period is, and why it matters, especially when it comes to the Omicron variant.

What is an incubation period, and why is it important?

When a virus infects your body, you usually don't experience symptoms right away—that's because it often takes time for the pathogen to replicate, or infect enough of your body's cells to make you sick. The period between your initial infection and your first symptoms of illness is called the incubation period. "Even though you're already infected with the virus, you may not know you're ill during the incubation period," epidemiologist Melissa Hawkins, PhD, director of undergraduate programs in the department of Health Studies at American University, tells Health.

While we still need more data to know for sure that Omicron has a shorter incubation period, its apparent shorter incubation period is important for a few reasons. First, is that it affects when people should test after an exposure. Because many COVID-19 tests can only detect COVID-19 proteins or antigens during certain points in the infection—usually when symptoms arise—testing too soon or too late could result in a false negative (and cause infected people to unknowingly spread the virus to others). "Timing does really matter and can make a difference between a positive and a negative test," Hawkins says. "If you were negative in the morning, you could be positive later that day or the next day."

That's why, as of October 2020—when the Delta strain was dominant—the CDC recommended vaccinated people test five to seven days after close contact with a person suspected to have COVID-19 (unvaccinated people were instructed to isolate immediately after exposure and watch for symptoms for 14 days). According to Hawkins, it's likely that health experts will start recommending testing closer to three days after a possible Omicron exposure to avoid unnecessary transmission. "We need to remember to update our thinking about waiting to be tested and how long to wait to avoid a false negative so we don't unknowingly transmit the virus to others," she says.

A shorter incubation period may also mean the virus becomes contagious faster, according to reporting from The Atlantic. Essentially, a shorter incubation period make a virus "much, much, much harder to control," Jennifer Nuzzo, DrPH, an epidemiologist at the Johns Hopkins Center for Health Security, recently told the outlet.

Are you contagious during the incubation period?

You certainly can be—we know that asymptomatic and presymptomatic people may be contagious in the absence of symptoms. In fact, you may be most contagious before you even have symptoms. A 2021 study published by Boston University School of Public Health in JAMA Internal Medicine found that people infected with COVID-19 were most contagious two days before and three days after symptoms appeared. According to Hawkins, most transmissions occur during that time frame, before people realize they are sick. Then, as the viral load decreases over time, spreading the virus to others becomes less likely.

The important takeaway here is that if you were in close contact with someone who tested positive for COVID-19 or has COVID-19 symptoms, it's best to stay home for a few days until you know for sure you're not infected. "Don't disregard a runny nose, sore throat, or headache, because those are symptoms, especially after an exposure," Hawkins says.

Does a virus' incubation period impact how long symptoms and contagiousness last?

In short, no: How long a virus incubates before causing symptoms doesn't affect how long the illness lasts—or how long people are contagious. Hawkins says variables like vaccination status, age, and pre-existing health condition are more important drivers. Still, two very similar vaccinated people may have totally different experiences with COVID-19. "We know some of the key risk factors that are likely to result in severe illness, but it's still a bit of a mystery why one person in a household may get sick and another person may not."

To protect yourself and others from infection, it's important to keep up the same safety measures health experts have been emphasizing since the beginning: Get vaccinated and boosted if you're eligible—the CDC recommends a booster shot two months after the Johnson & Johnson vaccine and six months after the second dose of an mRNA vaccine, like the two-dose Pfizer or Moderna shots.

Hawkins encourages continuing wearing a mask in public, and when you're gathering with people from outside your household. Because the Omicron strain spreads much more easily than other COVID-19 mutations, pay special attention to the quality of your mask and that you wear your mask correctly. Hawkins suggests double-masking in public if you only have access to cloth masks, and ensuring your mask snugly covers your nose and mouth at all times.

New strains and increased cases can be discouraging, but Hawkins says continuing evidence-based safety measures is the key to protecting both your physical and mental health. "There's a lot of room to be able to engage in the activities and see people we want to see, because we now have so many more effective tools to stay safe and mitigate the spread," she says.

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