MORE than half of people infected with the coronavirus caught it from someone who was not showing any symptoms, according to a study.
Health experts said controlling presymptomatic spread is crucial for keeping on top of the disease when lockdowns are eased.
Researchers in Ireland reviewed 17 global studies to estimate how much transmission occurs during the “presymptomatic” period.
They found between 33 and 80 per cent of cases caught the virus from people who would have had no idea they were even infected.
It takes an average of six days before a person develops the tell-tale signs of COVID-19, which includes a fever and persistent cough.
But in those days prior to symptoms, patients are infectious and can pass the deadly virus onto others.
Transmission is most likely one day before symptoms start, the team concluded, but can happen as early as three days before.
It hinges on the success of contact tracing, which involves identifying all individuals who have been in close contact with a confirmed case.
Lead author Miriam Casey, from University College Dublin, and colleagues found more than half of people infected with the coronavirus caught it from someone who wasn't showing any symptoms. The average incubation period was 5.8 days. The majority of transmission appears to occur between three days before symptoms start and two days after. They had collected data from 17 international studies.
One of the studies was carried out in Wuhan, the Chinese city where the killer virus originated. Data was also used from Hong Kong, Singapore and Italy. The studies had looked at the incubation period - the time between exposure to the virus and appearance of the first symptoms.
The average incubation period was 5.8 days, which is similar to the figure given by the World Health Organization (WHO).
People are not infectious and capable of passing the virus on to others during this whole period, just some of it.
The Dublin team said the majority of transmission appears to occur between three days before symptoms start and two days after.
Researchers used this information and a mathematical equation to create an estimate for how many cases are the result of presymptomatic spread.
In one study involving 137 people in Tianjin, a municipality in China, presymptomatic spread was estimated to be blamed for around 80.7 per cent of all cases.
A Wuhan study involving 12 people estimated presymptomatic spread was responsible for 33.7 per cent of cases, while it was predicted to be responsible for 36.3 per cent of all spread in a study of 240 people in Northern Italy.
Based on the evidence gathered, they estimated 56.1 per cent of transmission was from people before they showed symptoms.
But overall, a substantial amount of presymptomatic spread is contributing to the crisis, the authors warned. This backs previous estimates that pre-symptomatic people are responsible for up to 80 per cent of the spread of the virus.
The team said presymptomatic transmission alone can cause the virus to spread uncontrollably and 'sustain an epidemic of its own'. (Daily Mail)