Asymptomatic of Covid-19 spread deemed rare


TOKYO • Transmission of the novel coronavirus by people who don’t develop symptoms is “very rare”, the World Health Organisation (WHO) said, playing down concerns that these so-called silent spreaders might serve as a major conduit for the disease.

“It still appears to be rare that an asymptomatic person actually transmits onward to a secondary individual,” Maria Van Kerkhove, head of WHO’s emerging diseases and zoonosis unit, said at a briefing in Geneva. She said her comment, which reiterates the group’s previous position on so-called asymptomatic cases, is based on detailed reports of contact tracing from various countries.

More research is needed to confirm the roles that asymptomatic cases and pre-symptomatic cases — those who later go on to develop symptoms — play in spreading the disease. Uncertainty over how the virus behaves has hindered nations’ efforts to reopen battered economies.

“Comprehensive studies on transmission from asymptomatic individuals are difficult to conduct, but the available evidence from contact tracing reported by member states suggests that asymptomatically-infected individuals are much less likely to transmit the virus than those who develop symptoms,” the WHO said in guidance on the use of face masks that it issued last week.

Presymptomatic individuals, who develop a higher viral load just before the onset of symptoms, maybe infectious, the WHO said. The infection is spread by tiny droplets expelled when infected people sneeze, cough, speak or breathe.

Although the health organisation had said as far back as February that it did not see asymptomatic cases as a major cause of viral spread, other researchers have sparked concern that the virus would be difficult to contain because of this route of transmission. The New England Journal of Medicine, in an article dated May 28, warned that transmission of SARS- CoV-2 by seemingly healthy people is “the Achilles’ heel of Covid-19 pandemic control”.

Countries across the globe have been wary of relaxing social distancing guidelines and rigid travel restrictions, fearing that people without symptoms could spread the Covid-19 pathogen unchecked throughout communities.

“The asymptomatics are still important, particularly if you want to get levels of the virus down to very low levels of transmission,” said Peter Collignon, a professor of clinical medicine at the Australian National University Medical School in Canberra, who advises the Australian government on infection control.

Because identifying asymptomatic cases is so difficult, the US and other nations have struggled to implement adequate testing to gauge how widespread the disease has become. The Chinese city of Wuhan recently completed the testing of its entire population of 11 million in an effort to identify cases to avoid a resurgence of infections.

Van Kerkhove cited a number of reports from countries doing detailed contact tracing — in which asymptomatic cases and their contacts were followed — that found no evidence of secondary transmission. She said countries should focus on following symptomatic cases.

“If we actually follow all the symptomatic cases, isolated those cases, followed the contacts and quarantined those contacts, we would drastically reduce” transmission, she said. — Bloomberg