Omicron Variant's Incubation Period Is About 3 Days—What That Means

It appears to be shorter than the incubation period of other COVID-19 variants, which is important when it comes to testing and transmission.

In November of 2022, the Centers for Disease Control and Prevention (CDC) estimated that the Omicron variant of SARS-CoV-2 (the virus that causes COVID-19) was the dominant strain, accounting for nearly all new cases of COVID-19 in the United States.

As they have with previous strains of SARS-CoV-2, researchers have learned how Omicron differs from other strains of the virus. They have found that, along with its increased transmissibility and milder symptoms compared to previous variants, Omicron likely has a shorter incubation period than Delta and other strains.

A study conducted during high levels of Delta variant transmission reported an incubation period of 4.3 days, and studies performed during high levels of Omicron variant transmission reported a median incubation period of 3–4 days.

Here's what a virus' incubation period is and why it matters, especially when it comes to the Omicron variant.

what is an incubation period of a virus

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, told Health.

Omicron's apparent shorter incubation period is important for a few reasons. First, 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 said. "If you were negative in the morning, you could be positive later that day or the next day."

That's why in October 2020—when the Delta strain was dominant—the CDC recommended vaccinated people test five to seven days after close contact with a person who has COVID-19 (unvaccinated people were instructed to quarantine immediately after exposure and watch for symptoms for 14 days).

According to the CDC testing guidance updated in September 2022, people who were exposed to COVID-19 and do not have symptoms should wait at least 5 full days after the exposure before testing. If you test too early, you may be more likely to get an inaccurate result.

Second, a shorter incubation period may allow a virus to spread more quickly (enhanced transmission). Indeed, the Omicron variant spreads more easily than earlier variants of SARS-CoV-2. Anyone who has COVID-19—regardless of vaccination status, symptoms, or variant type—can spread the virus to others. In addition, Omicron can reinfect individuals, even if they have recently recovered from COVID-19.

How the Omicron Incubation Period Was Determined

In December 2021, the CDC first suggested that it could 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 SARS-CoV-2. (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 SARS-CoV-2.

More data suggesting a shorter incubation period for Omicron came from a May 2022 study looking at three Japanese public health centers. Study researchers were able to identify 77 eligible symptomatic patients infected with the Omicron variant for whom the date and setting of transmission were known. Based on their analysis, the researchers concluded that the incubation period for Omicron infection was approximately three days.

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, as well as to wear a mask when going out in public. "Don't disregard a runny nose, sore throat, or headache, because those are symptoms, especially after an exposure," Hawkins said.

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 said 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."

In August 2022, the CDC released revised COVID-19 guidance. The guidance continued to promote the importance of being up to date with vaccination to protect people against serious illness, hospitalization, and death.

Per the CDC, instead of quarantining if you were exposed to COVID-19, you should wear a high-quality mask for 10 days and get tested on day five. If you test positive for COVID-19, the guidelines recommend that you stay home for at least five days and isolate from others in your home, regardless of vaccination status.

The CDC's mask guidelines recommend wearing a mask when there is a lot of COVID-19 in your community, in public places where there are a lot of people around, if you visit someone who might get very sick from COVID-19, and in other scenarios.

Because the Omicron strain spreads much more easily than other SARS-CoV-2 mutations, pay special attention to the quality of your mask and make sure you wear your mask correctly. If you need a better fit or extra protection with cloth and disposable masks, the CDC suggests the following measures:

  • Wear two masks (disposable mask underneath AND cloth mask on top).
  • Combine either a cloth mask or disposable mask with a fitter or brace.
  • Knot and tuck ear loops of a 3-ply mask where they join the edge of the mask.
  • Use masks that attach behind the neck and head with either elastic bands or ties (instead of ear loops).

New strains of the virus and increased cases can be discouraging, but Hawkins said 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," Hawkins said.

A Quick Review

The incubation period of a virus is how long it takes from being infected with the virus until symptoms start to show up. The Omicron variant has an incubation period of about three days, which is shorter than the incubation period for other variants of the virus that cause COVID-19.

Incubation periods can affect when you should get tested after exposure and how easily the virus will spread. The best course of action to prevent COVID-19 is to follow the CDC guidelines, which means getting vaccinated and boosted, paying attention to how many cases are in your area, and masking when necessary.

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