Can asymptomatic people transmit covid-19? 1:45

. – The spread of covid-19 by an asymptomatic appears to be rare, Maria Van Kerkhove, technical leader of the World Health Organization (WHO) for the response to coronavirus and head of the emerging diseases and zoonosis unit, said Monday during a press conference in Geneva.

LOOK: WHO clarifies its sayings about the asymptomatic spread of covid-19

“From the data we have, it still seems rare that an asymptomatic person actually infects a secondary individual,” Van Kerkhove said Monday.

“We have several country reports that are doing very detailed contact tracking. Asymptomatic cases follow, contacts continue and they do not find secondary transmission from now on. It is very rare, and much of it is not published in the bibliography, “he said. “We are constantly looking at this data and we are trying to get more information from the countries to really answer this question. It still seems weird for an asymptomatic person to really catch it. ”

Van Kerkhove went on to describe how the new coronavirus, a respiratory pathogen, spreads through droplets, which can be released when someone coughs or sneezes.

“It passes from an individual through infectious droplets. If we were to really track all symptomatic cases, isolate those cases, track contacts, and quarantine those cases, we would dramatically reduce it. I would love to be able to give a ratio of how much transmission we would actually stop, but it would be a drastic reduction in transmission, ”he said.

Van Kerkhove also said that what appears to be asymptomatic cases of covid-19 often turn out to be cases of mild illness.

“When we actually go back and say how many of them were really asymptomatic, we found that many had really mild disease,” said Van Kerkhove.

“They are not symptoms of covid in quotes, which means they may not have developed a fever yet, they may not have had a significant cough or they may not have difficulty breathing, but some may have mild illness,” he said. “That said, we know that there may be truly asymptomatic people.”

Many people with covid-19 who may appear asymptomatic may actually have mild or atypical symptoms, or may be pre-symptomatic, Dr. Manisha Juthani, an infectious disease specialist and associate professor of medicine, told CNN on Monday. and epidemiology at the Yale School of Medicine.

READ: More than 370 workers at a pork plant in Missouri tested positive for coronavirus. They were all asymptomatic.

Pre-symptomatic refers to the early stages of an illness, before symptoms have developed, while asymptomatic may refer to having no symptoms during the course of an infection, a distinction Van Kerkhove hinted at during Monday’s briefing session. .

Juthani, who did not participate in the media briefing and has not reviewed the unpublished data cited by Van Kerkhove, said those findings are not necessarily inconsistent with other estimates of how the pre-symptomatic spread of the coronavirus can occur.

A study in April found that excretion of the virus, when people can infect others, could begin two to three days before symptoms appear. Additionally, the CDC estimates in planning scenarios that 40% of coronavirus transmission occurs before people feel sick.

“These patients were not asymptomatic,” said Juthani. Rather, they were “spreading the disease before becoming symptomatic.”

Overall, “these findings suggest that if we quarantine and contact symptomatic individuals, we can make a significant dent in the pandemic,” said Juthani.

Making this distinction between asymptomatic and pre-symptomatic infections is still important, but also between “paucisymptomatic infections”, which refers to having atypical or very mild symptoms, he said in a written statement distributed by the UK-based Science Media Center. Babak Javid, Principal Investigator at Tsinghua University School of Medicine in Beijing and consultant for infectious disease at Cambridge University Hospitals.

Detailed tracing of contacts from Taiwan, as well as the first European broadcast network in Germany, suggested that true asymptomatic people rarely transmit. However, those (and many other) studies have found that paucisymptomatic transmission can occur, and in particular, in the German study, they found that transmission often seemed to occur before or the day the first symptoms appeared, “said Javid in the statement.

“Other available data, from studies on several continents confirming that presymptomatic transmission occurs, would suggest that being well does not necessarily mean that one cannot transmit SARS-CoV-2,” Javid said in part. “This has important implications for the monitoring / tracing / isolation measures that are being implemented in many countries.”

CNN’s Michael Nedelman contributed to this report.