Global fears over a potential second wave of novel coronavirus infections are on the rise -- and there’s one phrase being thrown around -- the "reproduction rate", but what exactly does that mean?
The reproduction rate, or RO [pronounced R-naught] of a virus is a measure of its transmission, meaning the number of new infections generated by each case.
So, an RO rate of 1, means on average each infected person will infect one other person they come in contact with.
So what does this tell us? Well, a reproduction rate of less than 1 means that an outbreak is subsiding. As each infected person is transmitting the virus to fewer than one other person.
But an RO rate above 1 means the virus is spreading exponentially. This is because each contagious person is infecting more than one other person.
An RO of more than 1 is also an indication that hospitals and healthcare systems are vulnerable to becoming overwhelmed.
Let's put it into context -- during the initial outbreak of the new coronavirus in Wuhan, it was fast-spreading and the reproduction rate was estimated at around 2.5, according to a World Health Organization analysis.
Calculating the reproduction rate can be difficult, especially for a large country like the United States -- as the RO rate may vary from region to region. A lack of capacity for widespread testing can also make it difficult.
So what influences the coronavirus reproduction rate?
Population density is a major factor. The virus spreads much more efficiently in densely populated areas. So social distancing, school and business closures, and wearing face masks can all help drive the RO number down.
The characteristics of the virus itself is another major influence. This new coronavirus can spread more easily than other viruses. It has a median incubation period of about 5 days, compared with 2 days for flu, and people with no symptoms can spread the virus to others.
There has also been evidence of transmission just from close contact with an infected person even without virus particles spreading through coughing or sneezing.
Finally, the level of immunity in a population will also influence the RO -- either from previous infection with a virus or from vaccination rates. In this case, no one had immunity when the virus first emerged and there is still no approved vaccine.
So should we even be worried about the rising reproduction rate? The short answer is "yes." It could be an indication of a new surge of infections and it might influence the way countries decide to reopen.