Symptoms of Covid-19 infection appear in 42 hours after being exposed to SARS-CoV-2 virus, much earlier than previously thought, finds a study led by Imperial College London researchers and published on a pre-print server but not yet peer-reviewed. It also underlined the importance of masks in checking spread of Covid-19, as exposure to a single nasal droplet was found to be sufficient to cause the infection.
The World Health Organisation (WHO) says that Covid symptoms take around five to six days to appear after infection. However, UK researchers showed that symptoms start to develop very fast, on average about two days after contact with the virus.
The infection first appears in the throat; infectious virus peaks about five days into the infection and at that stage is significantly more abundant in the nose than the throat.
The findings also showed that the virus was detected first in the throat, significantly earlier than in the nose (40 hours in the throat compared to 58 hours in the nose). The levels were lower and peaked sooner in the throat.
Peak levels of virus were significantly higher in the nose than in the throat, indicating a potentially greater risk of virus being shed from the nose than the mouth.
This highlights the importance of proper face mask use to cover both the mouth and nose, the researchers said. The team also found that lateral flow tests (LFTs) are a reassuringly reliable indicator of whether infectious virus is present (that is, whether they are likely to be able to transmit virus to other people).
However, the LFT tests were less effective in picking up lower levels of virus at the very start and end of the infection.