London, Feb 3 (IANS): 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.

According to the World Health Organisation (WHO), Covid symptoms take around five to six days to appear after infection.

But the study led by Imperial College London 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The findings published on a pre-print server and not yet peer-reviewed also showed that the virus was detected first in the throat, significantly earlier than in the nose (40 hours in the throat compared to 58 hours in the nose)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.

"Our study reveals some very interesting clinical insights, particularly around the short incubation period of the virus, extremely high viral shedding from the nose as well as the utility of lateral flow tests with potential implications for public health," said Christopher Chiu, Department of Infectious Disease and the Institute of Infection at Imperial College.

In the trial, 36 healthy male and female volunteers aged 18-30 years, unvaccinated against Covid-19 and with no prior infection with SARS-CoV-2.

Participants were exposed to the lowest possible dose of virus found to cause infection, roughly equivalent to the amount found in a single droplet of nasal fluid when participants were at their most infectious.

Among the 18 infected participants, the average time from first exposure to the virus to viral detection and early symptoms (incubation period) was 42 hours, significantly shorter than existing estimates, which put the average incubation period at five to six days.

Following this period there was a steep rise in the amount of virus (viral load) found in swabs taken from participants' nose or throat.

These levels peaked at around five days into infection on average, but high levels of viable (infectious) virus were still picked up in lab tests up to nine days after inoculation on average, and up to a maximum of 12 days for some, supporting the isolation periods advocated in most guidelines.