People who are infected with coronavirus but show no symptoms only rarely transmit the disease, said a senior World Health Organization (WHO) official on Monday in comments that appear to contradict the common understanding of how COVID-19 spreads.
Until now, the scientific consensus has been that a significant amount of COVID-19 cases are asymptomatic, meaning the person shows no symptoms, but they can still spread the virus. This assumption has driven government policies aimed at containing the virus, resulting in stringent lockdowns based on predictions of large numbers of people spreading the virus without realizing they even have it.
But this assumption appeared to be challenged by the head of the WHO’s emerging diseases and zoonosis unit, Maria Van Kerkhove, on Monday – with potentially drastic consequences for global health policy.
“We have a number of reports from countries who are doing very detailed contact tracing. They’re following asymptomatic cases, they’re following contacts and they’re not finding secondary transmission onward. It is very rare – and much of that is not published in the literature,” said Van Kerkhove, speaking at a news briefing from the WHO’s Geneva headquarters.
“We are constantly looking at this data and we’re trying to get more information from countries to truly answer this question. It still appears to be rare that an asymptomatic individual actually transmits onward,” she added.
Isolate people with symptoms to stop spread
Van Kerkhove went on to explain that if the data is correct, health authorities should be able to “drastically reduce” the spread of coronavirus by identifying and isolating people with symptoms.
“If we actually followed all of the symptomatic cases, isolated those cases, followed the contacts and quarantined those cases, we would drastically reduce - I would love to be able to give a proportion of how much transmission we would actually stop - but it would be a drastic reduction in transmission,” she said.
Her statement implies that only people who showed symptoms of coronavirus would need to be isolated, potentially supporting the argument for reopening economies and allowing asymptomatic people to travel freely.
It also casts doubt on the existing health guidelines, which advocate social distancing as essential for public health.
For example, the US Centers for Disease Control and Prevention published a report on April 1 that cited the “potential for presymptomatic transmission” as a reason for the importance of social distancing - a transmission that Van Kerkhove is suggesting is rare.
Difference between asympomatic, pre-sympomatic, mild disease
Van Kerkhove made the comment about the transmission of cases after explaining the difference between asympomatic and pre-symptomatic individuals.
Van Kerkhove noted that measuring the number of truly asympomatic cases is difficult, but that the WHO had found that many people who had been reported as “asympomatic” actually had mild symptoms of the disease that had been difficult to detect.
“We hear from a number of countries that X number, X percentage of them are reported as not having symptoms, or that they are in their pre-symptomatic phase, which means that it’s a few days before they actually develop symptoms,” said Van Kerkhove.
“[However,] when we actually go back and we say how many of them were truly asymptomatic, we find out that many have really mild disease,” she added.
“They’re not quote-unquote Covid symptoms, meaning they may not have developed fever yet, they may not have had a significant cough, or they may not have shortness of breath – but some may have mild disease,” she said. “Having said that, we do know that there can be people who are truly asymptomatic.”
According to a scientist quoted by CNN, Van Kerkhove is only claiming that truly asymptomatic cases very rarely spread the disease, and not people with mild disease or pre-sympomatic cases.
“These patients weren't asymptomatic... [they were] spreading disease before becoming symptomatic.” said Dr. Manisha Juthani to CNN, referring to a study that found pre-symptomatic people could spread the disease two or three days before showing any symptoms.
Babak Javid, a consultant in infectious disease at Cambridge University Hospitals, who spoke to CNN agreed that recent studies suggested that asymptomatic transmission was rare, but presymptomatic transmission was more common
“Detailed contact tracing from Taiwan as well as the first European transmission chain in Germany suggested that true asymptomatics 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 appeared to occur before or on the day symptoms first appeared,” Javid said in a statement quoted by CNN.