Asymptomatic Spread is rare: WHO
The World Health Organization says it still believes the spread of the coronavirus from people without symptoms is “rare," despite warnings from numerous experts worldwide that such transmission is more frequent and likely explains why the pandemic has been so hard to contain. Maria Van Kerkhove, WHO's technical lead on COVID-19 said at a press briefing that many countries are reporting cases of spread from people who are asymptomatic, or those with no clinical symptoms. But when questioned in more detail about these cases, Van Kerkhove said many of them turn out to have mild disease, or unusual symptoms.
- Although health officials in countries including Britain, the U.S. and elsewhere have warned that COVID-19 is spreading from people without symptoms, WHO has maintained that this type of spread is not a driver of the pandemic and is probably accounts for about 6 per cent of spread, at most.
- Numerous studies have suggested that the virus is spreading from people without symptoms, but many of those are either anecdotal reports or based on modeling.
- Van Kerkhove said that based on data from countries, when people with no symptoms of COVID-19 are tracked over a long period to see if they spread the disease, there are very few cases of spread.
- We are constantly looking at this data and we're trying to get more information from countries to truly answer this question, she said. It still appears to be rare that asymptomatic individuals actually transmit onward.
- Symptomatic transmission refers to transmission from a person while they are experiencing symptoms.
- Data from published epidemiology and virologic studies provide evidence that COVID 19 is primarily transmitted from symptomatic people to others who are in close contact through respiratory droplets, by direct contact with infected persons, or by contact with contaminated objects and surfaces, the WHO says.
- The incubation period of the virus ranges between 5-14 days.
- Presymptomatic transmission is when a person spreads the disease before symptoms appear, and eventually develops symptoms themselves.
- On asymptomatic transmission, the WHO says it “refers to transmission of the virus from a person, who does not develop symptoms.
- There are few reports of laboratory-confirmed cases who are truly asymptomatic, and to date, there has been no documented asymptomatic transmission. This does not exclude the possibility that it may occur.
- Asymptomatic cases have been reported as part of contact tracing efforts in some countries.
- The US Centers for Disease Control and Prevention (CDC) is clear that both presymptomatic and asymptomatic transmissions of the virus are possible.
- The onset and duration of viral shedding and the period of infectiousness for COVID-19 are not yet known.
- It is possible that SARS-CoV-2 RNA may be detectable in the upper or lower respiratory tract for weeks after illness onset, similar to infections with MERS-CoV and SARS-CoV.
- However, detection of viral RNA does not necessarily mean that infectious virus is present.
- There are reports of asymptomatic infections (detection of virus with no development of symptoms) and pre-symptomatic infections (detection of virus prior to development of symptoms) with SARS-CoV-2, but their role in transmission is not yet known.
- Based on existing literature, the incubation period (the time from exposure to development of symptoms) of SARS-CoV-2 and other coronaviruses (e.g. MERS-CoV, SARS-CoV) ranges from 2-14 days.