CDC Study Says It Can Take About 3 Days for People Exposed to Omicron to Start Showing Symptoms (2024)

The Centers for Disease Control and Prevention (CDC) estimates that the Omicron variant accounts for all new cases of COVID-19 in the US. As Omicron continues to spread around the world, researchers have been learning more about how it differs from previous strains of SARS-Cov-2. They have found that, along with its increased transmissibility and milder symptoms than previous variants, Omicron may also have a shorter incubation period than Delta and other strains, says the CDC.

In December of 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 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 was not the only information suggesting a shorter incubation period for Omicron. A study from Japan, published in May of 2022 in the International Journal of Environmental Research and Public Health, looked at data from 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.

Another study from Spain, published in June of 2022 in Emerging Infectious Diseases, looked at a total of 622 cases of infection with the Omicron variant. The researchers found that the incubation period was a median of 3 days for both Omicron and Delta variants and found no differences within variants between vaccine status.

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.

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.

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," said Dr. Hawkins. "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 the latest CDC testing guidance, 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.

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, told the outlet.

Indeed the Omicron variant spreads more easily than earlier variants of the virus that cause COVID-19, including the Delta variant, according to the CDC. The agency warns that anyone with Omicron infection, regardless of vaccination status or whether or not they have symptoms, can spread the virus to others, and that Omicron can reinfect individuals, even if they have recently recovered from COVID-19.

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 Dr. 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," said Dr. Hawkins.

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. Dr. 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 of 2022, the CDC released new COVID-19 guidance. The guidance continues to promote the importance of being up to date with vaccination to protect people against serious illness, hospitalization, and death. Instead of quarantining if you were exposed to COVID-19, the CDC recommends that you 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 other scenarios.

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. Dr. 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 Dr. 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," said Dr. Hawkins.