Covid symptoms appear just two days after being exposed to the virus, study shows

Coronavirus symptoms appear just two days after being exposed to the virus – much earlier than previously thought, a study has found.

Under the landmark Human Challenge Programme, 36 healthy young adults were deliberately infected with Covid-19 at a London hospital.

The study found symptoms took on average just two days to appear, compared to previous estimates of around five to six days by the World Health Organisation (WHO).

Coronavirus symptoms appear just two days after being exposed to the virus – much earlier than previously thought, a study has found. Pictured: Lead author Professor Chris Chiu

The Government-backed research, led by Imperial College London, involved volunteers aged 18 to 29 who had never had Covid before.

They were kept in a quarantine unit for at least 14 days at the Royal Free Hospital and deliberately given a very small dose of coronavirus via nasal drops.

Of the group, just 53 per cent went on to develop Covid-19. The authors said the fact nearly half of the volunteers avoided the disease could hold help uncover why some peole have managed to avoid infection throughout the pandemic.

Experts found that coronavirus infection first appears in the throat and then symptoms peak on around day five, when levels of virus are significantly higher in the nose.

Sixteen of the group developed mild-to-moderate cold-like symptoms, including a stuffy or runny nose, sneezing and sore throat, but none fell seriously ill.

Lead author Professor Chris Chiu said: ‘Very shortly after they are given the virus, within two days, people start to have detectable virus, first in the throat but then in the nose, which rises very quickly.’ He said that, among those who had no symptoms, some still have very low levels of viral detection, suggesting there may be an immune process going on to suppress the infection.

The study found symptoms took on average just two days to appear, compared to previous estimates of around five to six days by the World Health Organisation (WHO)

The study also found that lateral flow tests are highly effective at picking up the virus.

Co-author Dr Andrew Catchpole, chief scientific officer at hVIVO, said the findings would also apply to the Delta and Omicron variants even though the study used the ‘original’ Covid-19 variant.

He said: ‘This shows the potential of the virus to replicate very, very quickly.

‘We haven’t done Delta or Omicron in challenge studies so all we can do is speculate what might be seen.

‘What we do know – from what’s seen in the field – is that those viruses do… transmit very quickly and replicate very quickly, so we would expect there to be a lot of transferrable data between the two.’ The study also found that high levels of viable (infectious) virus could still be picked up in lab tests for up to nine days, and 12 days in some cases.

Peak levels of virus were significantly higher in the nose than in the throat, suggesting a potentially greater risk of virus being shed from the nose than the mouth, it said.

The researchers said this highlights the need for people to use facemasks properly that cover both the mouth and nose.

Professor Sir Jonathan Van-Tam, deputy chief medical officer for England, said: ‘Scientifically these studies offer real advantage because the timing of exposure to the virus is always known exactly, therefore things like the interval between exposure and the profile of virus shedding can be accurately described.

‘This important study has provided further key data on Covid-19 and how it spreads, which is invaluable in learning more about this novel virus, so we can fine-tune our response.

‘Challenge studies could still prove to be important in the future to speed the development of ‘next-generation’ Covid-19 vaccines and antiviral drugs.

‘This data underline just how useful a tool lateral flow tests can be to pick up people when infectious and the importance of wearing a face covering in crowded, enclosed spaces.’