Is omicron variant the fastest spreading virus? It depends on the metric used for measuring

As the omicron variant of COVID-19 rapidly made its way around the world — infiltrating 89 countries within three weeks of first being identified in late November — it became the fastest-spreading variant since the start of the pandemic.

Social media users attempted to quantify its place among viruses throughout history.

"Omicron is now the fastest-spreading virus known to humankind," began one three-minute Facebook video. It was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

There is more than one way to quantify virus’ spread: One is to measure how easily a virus is transmitted from person to person, and another is to look at how quickly it spreads across the world. When it comes to the former, measles is the fastest. For the latter, it’s omicron.

Measles is the most contagious virus when it comes to transmission between people, according to the Centers for Disease Control and Prevention. Each person who contracts it can infect 12 to 18 other people, in a population not protected by vaccines and other preventative measures.

The omicron variant ranks as a close second, though: Each person who contracts omicron can infect between 7 and 14 people in an unvaccinated, unprotected population, studies show.

One term scientists use to quantify infectious disease outbreaks is R0, pronounced "R naught," and called the basic reproduction number. It indicates the average number of times one person will transmit the virus to someone else in a population where no one has been exposed to or vaccinated against the disease, and is therefore a more theoretical measure.

The basic reproduction number for measles is between 12 and 18. For COVID-19, the basic reproduction number has increased with each strain of the virus. With the original strain, each sick person likely would infect between two and three others; with the delta variant, between five and seven; and with omicron, most estimates put it at 1.5 to 2 times more than delta. That means with omicron, each sick person infects between 7 and 14 people.

Another mathematical measure for the spread of infection is called Rt, which is the effective reproduction number. It takes into account factors such as vaccination levels and immunity from prior exposure and "is likely lower than the basic reproduction number," according to the Health Lab. The Rt value is more of a real-time representation of what a virus’ infection rate is.

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The effective reproduction number is measured during an ongoing pandemic, and for COVID-19, it also has varied for each strain of the virus. With the original strain, each sick person would infect around 2.5 others; with the delta variant, it was 3.5 to 4; and with omicron, scientists have estimated it between 3.7 and 4.2.

When measuring how fast a virus spreads around the world, "Omicron is certainly the most rapidly spreading virus among the ones we have been able to investigate at this level of detail," William Hanage, an associate professor of epidemiology at Harvard T.H. Chan School of Public Health, told El Pais, a newspaper in Spain, in an interview.

Hanage told PolitiFact that’s because even though each measles infection causes more secondary infections in an unprotected population, it takes longer than omicron to do so. The time it takes one person infected with the omicron variant to infect another person — called the generation time — is shorter than with measles, in a non-immune population.

The generation time is about 12 days for measles and four to five days for omicron, El Pais reported.

"As a result of omicron's shorter generation time — and despite its lower R0 — the numbers of omicron cases climb more rapidly per unit of time than measles," Hanage said.

A Facebook video said, "Omicron is now the fastest-spreading virus known to humankind."

When it comes to how quickly viruses spread across the world, omicron is the fastest, experts say.

When it comes to how viruses are transmitted from person to person, measles is the most contagious. Each person who has measles can infect 12 to 18 other people, in a population not protected by vaccines and other preventative measures, according to the CDC. 

But the omicron variant of SARS-CoV-2 is a close second. Estimates put it at 1.5 to 2 times more transmissible than the delta variant, which saw each sick person infecting between five and seven others. So each person who contracts the omicron variant can infect between 7 and 14 people in an unvaccinated, unprotected population.

We rate this claim Mostly True.