The minister for Covid vaccines deployment, Nadhim Zahawi, told BBC Radio 4 this week that coronavirus “is the most infectious respiratory disease, that is aerosol transmitted, that man has ever had to deal with”.
But it looks like he’s wrong.
Epidemiologists often use the concept “R0” or the “reproduction number” to describe how infectious a disease is. The number that follows the R tells us how many people the average person who has the disease is likely to infect.
Human coronaviruses, including SARS-Cov2 (the one responsible for the pandemic), have basic reproduction numbers between 0.5 and 8.0, according to a review of the academic literature published in the journal Nature in March this year.
That means that, without interventions like social distancing or vaccines, we’d expect the average person with one of these viruses to pass it on to up to eight others.
The same article shows that R numbers for different influenza viruses can vary from 1.0 to 21.0. Meanwhile the varicella zoster virus (which most often causes chickenpox, but can cause a type of pneumonia) scores 1.2 to 16.9, and measles between 1.4 and 770. Though in the case of measles, the more commonly-cited range in scientific literature is a reproduction number between 12 and 18.
For all three virus groups, there is evidence of aerosol transmission, which means that they can spread through tiny droplets that contain infectious particles.
In other words, when we look at basic reproduction numbers, Covid-19 is not the most infectious aerosol-transmitted respiratory disease humanity has encountered.
It could be that the vaccines minister had another measure of infectiousness in mind – the “household secondary attack rate”, or SAR. This refers to the probability that someone already infected with a disease will pass it on to people in their household.
But again, even the highest estimate of the coronavirus SAR according to the round-up in Nature (38.2 per cent) is lower than the ranges for measles (52 to 85 per cent) and varicella zoster virus (61 to 78 per cent).
Update: 22 July
After we published this story, the Department of Health got in touch to tell us that Mr Zahawi – having made a near-identical claim in the House of Commons about the infectiousness of Covid – has now had his comments corrected in Hansard, the official record of parliamentary proceedings.
The vaccines minister initially told MPs: “I remind the House that this is probably the most infectious respiratory virus known to humankind, with aerosol transmission, so we need to ensure that we are careful as we head into step 4.”
He later wrote to Hansard to say that “An error has been identified in my response” and “the correct response should have been […] I remind the House that this is probably one of the most infectious respiratory viruses known to humankind, with aerosol transmission, so we need to ensure that we are careful as we head into step 4 [his emphasis].”