Question: I was very confused when I read yesterday that WHO is suggesting that asymptomatic transmission is rare. I thought we had evidence that asymptomatic spread was a major concern. Is there new research or guidance on this topic?
Answer: Yesterday during a press conference and in response to a reporter’s question WHO’s Dr. Maria Van Kerkov made the statement that asymptomatic transmission is “very rare.” I was very confused and slightly shocked by this statement and the media reports about it. It flies in the face of what we currently know (okay, that can happen in science), and was said without presenting any data to back it up (not okay, that’s bad public health communication). In short, please disregard what WHO said on this topic yesterday. And now for a more detailed response:
A snapshot of what we currently know: There is a growing body of evidence that asymptomatic spread — or at least presymptomatic spread — is of real concern. Again, asymptomatic means a person is infected but never develops symptoms (or symptoms of any note) while presymptomatic means that they are infected but haven’t yet experienced symptoms. This study published recently in Nature found that viral loads are very high early stages of the infection. Meanwhile, a study still awaiting peer review estimates that the proportion of transmission that was attributed to the presymptomatic period was 48% in Singapore and 62% in Tianjin, China. CDC offers this synthesis of the early evidence here, stating “Recent epidemiologic, virologic, and modeling reports support the possibility of SARS-CoV-2 transmission from persons who are presymptomatic or asymptomatic.”
Challenges in public health communication: In response to the confusion, this morning WHO hosted a live Q&A to address the concerns. And the upshot is, WHO screwed up its communication yesterday. No guidance has changed. Dr. Maria Van Kerkov admitted error in using the words “very rare” and the new data she was basing her statement on are not robust enough to make such a statement or generalization. She clarified that yesterday’s statement was made with regard to people who are truly asymptomatic rather than presymptomatic. And she stated, “Some estimates of around 40 percent of transmission may be due to asymptomatic, but those are from models, and so I didn’t include that in my answer yesterday but wanted to make sure that I covered that here.” For further discussion on this issue, health expert Andy Slavitt offers an interesting take in his twitter thread.
Finally, our knowledge is not static and as we learn more, we adjust and even change our guidelines based on our evolving understanding. There’s still so much we don’t know and it’s quite possible, even expected, that we will have some major shifts in our understanding, including our understanding of asymptomatic cases and transmission. But as of June 9th, we don’t have any evidence that changes our understanding of the lay of the land pre-WHO press conference of June 8th. Please keep wearing your mask and please keep physical distance… please keep doing all the things to help keep us protected!