What is R0?

There’s a lot riding on the ‘R’ – or reproductive number – of the novel coronavirus (SARS CoV2) which causes COVID-19. As the number rises and falls, ‘lockdowns’ and restrictions will tighten or ease.

It could determine how quickly people return to work, whether you go on holiday, and when to reopen schools. Crucially, it might be the reason we find ourselves in some form of lockdown again six months down the road.

The reproductive number is the average number of people who will contract a transmissible infection from one person with the disease. If that number is 1, it means someone with the virus will pass it to one person. If it’s two, each infected person will pass it to two people, who will each give it to two more people, and so on.

If the number is greater than 1, the number of people with the virus will rise; if it’s less than 1, the number is in decline.

Early in the coronavirus outbreak, experts were keen to know the R₀ (pronounced ‘R nought’) – the number of people infected with the virus where there is no immunity. This is a measure of how infectious the disease is when no measures are taken to prevent its spread and nobody has recovered from the illness or received a vaccine.

However, while R₀ represents the ‘natural’ infectiousness of a new bug, the effective reproductive number (sometimes written as R_e or simply as R) tells us how many new cases are likely to be caused by each existing case. That is, how many people will catch the virus now, given that some people might be immune and/or social distancing measures are in place.

It’s inaccurate to talk about the R₀ of coronavirus now that the world has responded with containment measures and some people have developed immunity. However, as politicians had already got their head around the term R0, they seem reluctant to ditch in favour of the more accurate ‘R’.

This is how Angela Merkel explained the effect of a higher #covid19 infection rate on the country’s health system.

This part of today’s press conf was great, so I just added English subtitels for all non-German speakers. #flattenthecurve pic.twitter.com/VzBLdh16kR

— Benjamin Alvarez (@BenjAlvarez1) April 15, 2020

This is a bugbear for Dr Peter English, a public health consultant and member of the Vaccines Today Editorial Board. In his blog on the topic, Dr English also explains why keeping R as low as possible could end the pandemic. While R < 1 means the outbreak is shrinking, keeping it to 0.5 is significantly better than holding it at 0.9. The former would see cases fizzle out by August; the latter mean we are living with new cases every day until March 2021.

In reality, it is possible that R will bounce back up and in the months ahead, prompting tweaks in public health policy along the way. Germany and South Korea are among the many countries to have brought their R below one, only to see it rise above one after restrictions are eased.

We are all epidemiologists now

During the global financial crisis a decade ago, public interest in debit, deficits and bond markets increased. Financial literacy grew; suddenly we were all economists. Perhaps we can hope for greater understanding of infectious diseases and how they spread.

One of the knock-on effects of the growing interest in R₀ is (a) that we can compare the infectiousness of SARS CoV2 to other viruses and (b) we are witnessing in real-time, how immunity and other measures can contain viruses.

The most common comparison you’ll find in the media today is between the novel coronavirus and influenza. R₀ for coronavirus was estimated early in the pandemic to be between 2 and 2.5. Flu is roughly half that. The H1N1, for which we initially had no vaccine when it first emerged, is estimated to have an R₀ of 1.2-1.6.

SARS (R₀ = 4) and MERS (R₀ = <1) which are also coronaviruses, illustrate how variable this class of viruses can be in their ability to spread.

The new coronavirus also has a higher R0 than Ebola where each person with the virus infects two others, in the absence of disease control measures. In the 1980s, HIV’s R₀ was estimated to be 4 – although the R_e is now far lower thanks to better understanding of how the disease spreads and the development of antiviral medicines.

But what about the infectious diseases for which we currently have vaccines? What is their R₀?

• Smallpox has an R₀ of 3
• Polio has an R₀ of 4-6
• Mumps has an R₀ of 10-12
• Chickenpox has an R₀ of 10-12
• Pertussis has an R₀  of 15-17
• Measles has an R₀  of 16-18

This explains why measles elimination requires such a high level of immunity in the population. WHO and national health experts say two doses of measles-containing vaccines (MMR or MMRV) for 95% of the population are needed to stop the spread of the disease. In the absence of high uptake of the vaccine, even in small pockets of the community, outbreaks will occur.

The (relatively) good news is that SARS-Cov2 is much less transmissible than measles. We won’t need 95% immunity to stop its spread. And other containment measures, such as social distancing, clearly have a significant impact on its effective R. But, unlike MERS, it is likely that continued outbreaks will occur if the virus is given the opportunity.