Fact Check-No evidence Omicron BA.5 is more infectious than measles or is 'the most infectious virus known'

Scientists say there is no evidence that the BA.5 version of the Omicron coronavirus variant is more transmissible than measles, or that it has a basic reproduction number, or “R-naught” (R0), of 18.6, which would be greater than that of measles. While BA.5 appears to be spreading faster than other Omicrons still circulating, experts say that BA.5 is unlikely to be one of “the most infectious viruses known to man,” as some have claimed.

Calculations recently published online (here) and (here) suggested that the R0 of BA.5 is 18.6, which would be nearly six times that of the original strain of the SARS-CoV-2 virus that emerged in 2019 and was estimated to have an R0 of about 3.0 to 3.3.

Basic reproduction number, or R0, is an estimate of the "number of secondary cases generated by a typical infectious individual when the rest of the population is susceptible (ie, at the start of a novel outbreak)" (here).

The calculation that BA.5 has an R0 of 18.6 was made by the article's author Adrian Esterman, a professor of biostatistics and epidemiology based in Australia (here), who implies that BA.5 has displaced measles -- with its R0 of 18 -- as the virus with the highest known basic reproduction rate, writing: "This is similar to measles, which until now was our most infectious viral disease."

The claim made its way into online articles (here) , (here), and has been repeated and shared more than 20,000 times on Twitter.

In sharing Esterman's result, one Twitter user remarked, "The latest Covid subvariants have an Ro value of 18.6. If you think it's over, guess again" (here) and another said, "The Omicron variants #BA4 #BA5 have an R0 of 18.6. In 2020, the R0 was around 1-3 or 4 at its worst. #COVID19 is now the most infectious disease in human history. Buckle up. This is it." (here).

But many Twitter users, including professional biostatisticians and epidemiologists, questioned the result and how it was calculated. "This statement about R0 is almost certainly incorrect and very frustrating to see going viral" (here), said one.

Another said, "Strikes me that there is some misunderstanding about the relationship between R0, growth advantage, and R_eff - and a bit of gen time as well" (here).

A third said, "Still seeing claims that latest COVID variants have R0 (i.e. R in fully susceptible population) of almost 20. But the same logic would lead to (incorrect) conclusion that seasonal flu has an R0 in the hundreds, if not thousands" (here).

More examples can be seen (here) , (here) and (here).

Esterman’s calculation is based on multiplying two very different types of measurement, scientists said. One is a context-dependent comparison -- BA.5’s “growth advantage” over the BA.2 version of Omicron, or BA.2’s growth advantage over BA.1 -- while the other is a more intrinsic property of the virus and how it would behave on a theoretical level playing field.

In his article, Esterman states the R0 for BA.1, the first version of Omicron to spread globally, is 9.5, citing a review of mostly South African studies (here) done soon after Omicron's emergence in that country. In addition to R0, the authors of that study calculated an average effective reproduction rate (R_eff) of 3.4.

Effective reproduction rate is "the expected number of new infections caused by an infectious individual in a population where some individuals may no longer be susceptible," and is considered a better reflection of how fast an epidemic will actually grow in a real setting (here) and (here).

Esterman then writes, "BA.2, which is the dominant subvariant in Australia at the moment, is 1.4 times more transmissible than BA.1, and so has an R0 of about 13.3." He further adds, "a pre-print publication from South Africa suggests BA.4/5 has a growth advantage over BA.2 similar to the growth advantage of BA.2 over BA.1. That would give it an R0 of 18.6," citing an analysis (here) from the period when BA.4 and BA.5 infections were rising in South Africa while BA.2 infections slowed.

Natalie Dean, an associate professor of biostatistics and bioinformatics, and of epidemiology at the Rollins School of Public Health of Emory University, tweeted about the result, "Just because something has a 50% *growth advantage* in a population does not mean it is 50% more *transmissible.* Some (or most) of that growth advantage may come from immune evasion." (here)

"So if each new variant has a 50% growth advantage, it does not mean that R0 keeps increasing by 50%. And while new variants are more transmissible, R0 is not up to 18 (measles territory). The reality is that R0 is tricky to apply to our current situation. Interpret with care!" (here)

Multiple factors can give one variant an advantage over another, Dean told Reuters. They include a change in the virus that makes it inherently more transmissible, but another is that it can evade recognition by the immune system in people who have been exposed to previous variants or to vaccines based on older versions of the virus.

“The R0 is just focused on the transmissibility part, but unless you untangle the two, you can’t tell how much of the advantage is due to [immune evasion],” she explained. With the assorted Omicron lineages, “a large chunk” of the advantage they have relative to one another and to previous variants comes from immune evasion, she said.

“You don’t even need an increase in transmissibility to explain the advantage,” Dean noted. But in multiplying R0 by relative growth advantage, “at each step, you’re attributing those advantages [only] to increased transmissibility.”

It would be difficult to calculate a true R0 for BA.5 now because the world has such mixed levels of exposure and vaccination, Dean said. “If you put BA.5 and the Wuhan strain in the same population, we don’t know what would happen. BA.5 might do better, but not six times as much.”


False. The basic reproduction number of the BA.5 Omicron variant was miscalculated as 18.6, scientists say, and therefore it is not greater than that of measles, and BA.5 is not the most infectious viral disease known.

This article was produced by the Reuters Fact Check team. Read more about our work to fact-check social media posts here .