(HOI) -- An epidemiologist with the World Health Organization says emerging studies show true asymptomatic spread of Covid-19 is "rare."
"It still appears to be rare that an asymptomatic individual actually transmits onward," Maria Van Kerkhove said.
On its face, it appears to be a breakthrough in understanding the virus' transmission, but other scientists were quick to point out the difference between asymptomatic and pre-symptomatic.
Pre-symptomatic refers to patients who carry the disease but don't show even mild symptoms yet. Asymptomatic refers to patients who carry the disease and never show any symptoms.
Since asymptomatic and pre-symptomatic cases can be hard to distinguish, the usefulness of this observation could be hard to quantify. Van Kerkhove also said follow-ups with people previously thought to be asymptomatic revealed, "many have really mild disease," which would mean they were actually pre-symptomatic.
Dr. Ashish K. Jha, director of Harvard's Global Health Institute, said on Twitter that Van Kerkhove's statement about transmission does not apply to pre-symptomatic people.
"If folks without symptoms truly 'very rarely' spread virus, would be huge," he writes. "But such a statement by @WHO should be accompanied by data."
Van Kerkhove said tracing symptomatic cases should be a priority for health departments around the world.
"We know this is a respiratory pathogen, it passes from an individual through infectious droplets," she said. "If we actually followed all of the symptomatic cases, isolated those cases, follow the contacts and quarantine those contacts 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."
You can watch her remarks in full in the video below. Skip to around 31:53.
Here is a transcript.
Question: I know the WHO has previously said there's no documented cases of [asymptomatic transmission]. We have a story out of Singapore today saying that at least half of the new cases they're seeing have no symptoms. I'm wondering if it's possible that this has a bigger role than the WHO initially thought in propagating the pandemic and what the policy implications of that might be.
"There's a couple of things in the question you just asked. One is the number of cases that are reported that are being reported as asymptomatic. 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 it's a few days before they actually develop severe symptoms. In a number of countries, when we go back and discuss with them: 1) how are these asymptomatic cases being identified, many of them are being identified through contact tracing, which is what we would want to see. You have a known case, you find your contacts, they're already in quarantine hopefully, and some of them are tested and then you pick up people who have no symptoms or even mild symptoms. The other thing we're finding is when we go back and say, 'how many of them were truly asymptomatic' we find that many have really mild disease, they're not 'Covid' symptoms, meaning they may not have developed fever yet. They may not have a significant cough. They may not have shortness of breath, but some may have mild disease. Having said that, we do know there can be people who are truly asymptomatic and PCR-positive.
"The second part of your question is, what proportion of asymptomatic individuals actually transmit. The way we look at that is these individuals need to be followed carefully over the course of when they're detected and looking at secondary transmission. We have a number of reports from countries that are doing very detailed contact tracing. They're following asymptomatic cases, they're following contacts and they're not finding secondary transmission onward. It's very rare. Much of that is not published in the literature. From the papers that are published, there's one that came out from Singapore looking at a long term care facility, there are some household transmission studies where you follow individuals over time and you look at the proportion of those that transmit onward. 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.
"What we really want to be focused on is following the symptomatic cases. If we follow all the symptomatic cases, because we know this is a respiratory pathogen, it passes from an individual through infectious droplets, if we actually followed all of the symptomatic cases, isolated those cases, follow the contacts and quarantine those contacts 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. If we could focus on that I think we would do very very well in terms of suppressing transmission. But, from the data we have, it seems to be rare that an asymptomatic person actually transmits onward to a secondary individual."Maria Van Kerkhove, WHO