How do we measure transmission?
During a COVID-19 outbreak, individuals with the virus can spread the infection to a number of other people in their community. The average number of people that a COVID-19 case spreads infection to is called the ‘reproduction number’. This number allows us to understand the spread and control of infectious diseases like COVID-19, and is an essential ingredient of the models and simulations that are now informing government control policies.
For a COVID-19 epidemic to grow, each case must on average create more infection than there was before. In other words, the case reproduction number must be bigger than 1. There is evidence that at the start of a COVID-19 outbreak, when there are few control measures in place and no one has previously been infected, the reproduction number (called R0 or “R-nought”) can be between 2 and 3. The exact value of R0 might be higher or lower among different population groups or in different locations, depending on how exactly people interact.
If the reproduction number is above 1, we will see ‘exponential growth’ at the start of an outbreak. If the first case infects 2 others (i.e. R0 = 2), and these new cases each infect 2 others, and these new cases infect 2 others, we would expect to see 8 new cases appear after only three generations of transmission. There is on average around a 4-day gap between each of these generations, which means that if R0 = 2, the number of cases arising each day would double every four days; this period is called the ‘doubling time’ of the infection.
How can we stop the spread of COVID-19?
R0 not only determines how easily a disease spreads, it also shows how much effort is required to control it. If R0 is 2, we need to reduce transmission by 50% to get the reproduction number below the critical value of 1; if R0 is 3, we need to reduce transmission by 66%.
There are no vaccines or clinically validated antiviral treatments for the new coronavirus (COVID-19 virus), so to reduce the reproduction number enough to stop the epidemic growing, we must limit contacts between healthy and infectious people. There are several options that can help do this, ranging from isolating those showing symptoms (and tracing the people they have come into contact with and isolating them too) to widespread social distancing. There is evidence that the intensive ‘lockdown’-type control measures introduced in China in late January led to a substantial reduction in transmission, and there has also been a reduction in transmission in other countries that have introduced similar measures.
When can control measures stop?
Lockdowns are effective, but also highly disruptive, socially and economically. Ideally, it will be possible to identify and implement a combination of measures that are less disruptive, but which are still effective at stopping the disease from spreading. If control measures are lifted however, there is a risk of a second wave of infections.
If the coronavirus cannot be eliminated – as now seems likely – the only way COVID-19 transmission will stop in the absence of control measures is if large numbers of people obtain immunity to the virus, either because they have been infected during the pandemic or because a vaccine has been developed. A population where the disease cannot spread because there are too few susceptible individuals for the reproduction number to be greater than 1 is said to have “herd immunity”. If R0 = 2 when the coronavirus is introduced into a population where everyone can catch the disease, then at least 1/2 (50%) of the population must be immune or vaccinated for herd immunity to occur. If R0 = 3, that fraction rises to 2/3 (66%). In the coming months, COVID-19 is likely to be suppressed by various forms of social distancing to reduce contacts, supported by natural immunity. The longer-term goal is to create herd immunity by vaccination.