England’s chief medical officer has warned the second wave of coronavirus cases could be “more severe” than the first, but what causes a second wave?

Professor Chris Whitty said the virus may spread rapidly if the second peak arrives in winter and warned the R number - the rate of infection - must be kept below one.

An R number of one means an infected person passes the illness on to one other person, if it was two, one person with coronavirus would pass it on to two others, meaning Covid-19 would spread exponentially.

Speaking in an online Gresham College lecture, Prof Whitty said: “We need to make sure that R does not go back above one because if not we will go back to a second wave.

“It is entirely plausible for a second wave to actually be more severe than the first if it is not mitigated.”

  • What could cause a second wave in coronavirus cases?

A secondary pandemic wave may occur if lockdown restrictions are lifted and international travel restarts.

As there are remaining infections within the population, this will increase the R number above one – causing an exponential growth in new infections.

Without any restrictions in place, an outbreak will begin to grow as long as the average number of people infected by someone with the virus Is over one.

In South Korea and China, the loosening of restrictions has triggered a resurgence in new cases.

A new cluster of coronavirus cases emerged in the country’s capital Seoul, sparking fears of a second wave of infections in the East Asian country.

China has started testing millions of people to catch new infections and South Korea has dispatched several thousand police officers in a renewed push for contact tracing.

South Korea has experienced a resurgence in cases. Credit: AP
  • What other pandemics have caused a second wave?

Both the 1918 Spanish flu and the 1957-58 Asian flu reappeared with a second wave that was more severe than the first one.

  • What happened during the Spanish flu second wave?

The second wave of the Spanish flu resulted in catastrophic global losses, with deaths reaching into the tens of millions and produced the majority of infections and deaths associated with the pandemic.

While the timing and number of waves was not consistent globally, the pandemic is generally viewed to have had three distinct waves: the spring of 1918, the autumn of 1918, and the winter of 1918–1919.

The appearance of these waves has been due to different reasons, with the presence of large proportion of susceptible populations following the first wave, or a possible virus mutation, noted as possible drivers.

A second wave of the virus could prove to be more deadly. Credit: PA
  • What evidence is there that people who had the disease once will not get it again? If this is the case, will this result in less deaths in a second wave?

The evidence is very scarce on whether those infected develop immunity to the virus.

Modelling studies have been assuming that recovery from Covid-19 infection means immunity, but it remains an open question.

If this is the case, the pool of susceptibility to infection in the second wave will be less, but we're not able to say this for sure, as we currently don't know how much of the infection is asymptomatic.

Coronavirus: Everything you need to know: