We came across this claim on Twitter:

According to the tweet, the new Omicron variants BA.4/5 have a reproduction rate (R0) of 18.6, compared to the R0 of the original Covid-19 virus which was around 1-3 or 4 at its worst. This would make the current variant of Covid-19 the most infectious disease in human history.

The tweet was accompanied by a graph showing the reproduction rates of major infectious diseases, with the most infectious disease before the BA.4/5 variants of Covid-19 being measles with a R0 of 16-18.

For context, the reproduction rate of a disease refers to the expected number of cases directly generated by one case in a population where all individuals are susceptible to infection. The definition assumes that no other individuals are infected or immunised (naturally or through vaccination). If this claim is true then, an individual infected with the BA.4/5 variant of Covid-19 could infect 18-19 people where someone with the Alpha variant would have only infected 3-4.

When we conducted a check on this claim however, we found that various public health experts had questioned the claim and noted that R0 is not an accurate gauge of transmissibility. As previously stated, R0 is only fully accurate in a “naive” population, where there is a lack of immunity or immunologic memory to a disease.

A fact check conducted by Reuters on this claim found that R0 is not an accurate indicator of infectiousness and multiple factors can give one variant an advantage over another. Factors could include a change in the virus that makes it inherently more transmissible, or the ability of a virus to 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.

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. 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, and not R0. As a result of the many factors in play, it would be difficult to calculate a true R0 for BA.4/5 now because the world has such mixed levels of exposure and vaccination.

The claim that the Omicron BA.4/5 are the most infectious viruses known to man is therefore false. R0 is not an effective indicator of virus transmissibility in a vaccinated population and it does not take immune evasion into account when gauging transmissibility.