Why the f😮ck is the WHO saying this? I get that they want to discourage the “oh, let's just let everyone get infected” ‘policy’, but “no evidence” at all that recovery confers immunity? Really? Not “insufficient evidence” or “no definite proof”? •1/21
Let's see… First, we have evidence that macaque monkeys infected with SARS-CoV-2 could not be reinfected after they had recovered: biorxiv.org/content/10.110… — so, yeah, this is on monkeys and the paper hasn't been peer-reviewed, so “insufficient evidence”… but not “none”. •2/21
Well, of course, we weren't going to deliberately inoculate humans with SARS-CoV-2 after they recovered. But amazingly, this has been done in the past with related coronaviruses, in particular HCoV-229E: ncbi.nlm.nih.gov/pubmed/2170159 •3/21
— so, after been infected a first time with HCoV-229E, a small group of volunteers were reinoculated a year later, and while some could be reinfected, none developed any symptoms. Similar results in ncbi.nlm.nih.gov/pubmed/6319590 •4/21
Again, this is insufficient evidence, because it's a related virus, not SARS-CoV-2 (though one would expect endemic coronaviruses to elicit a weaker immune response because they have evolved to adapt to humans and cause lesser symptoms). Inconclusive—but not “no evidence”. •5/21
(There is even some limited evidence that there may be cross-strain immunity between coronaviruses of the same genus: ncbi.nlm.nih.gov/pubmed/22188723 — and some virologists have suggested that it might explain the Covid-19 asymptomatics. See also ncbi.nlm.nih.gov/pmc/articles/P…) •6/21
Then we have direct evidence that Covid-19 elicits a range of immune responses in humans even in mild to moderate cases: ncbi.nlm.nih.gov/pubmed/32284614 — and antibody testing carried out in various places has returned significant seropositive rates. •7/21
(There is also some preliminary evidence that antibodies from the plasma of convalescing patients can be used to treat more severe cases, with potentially promising results: pnas.org/content/early/…) •8/21
But perhaps the most significant evidence of immunity is the negative one: reports of Covid-19 reinfections are scarce and it's not clear whether any of them are true reinfections or variations in viral load, as in nature.com/articles/s4158… •9/21
The reason for these apparent reinfections is confusing and needs to be further studied, but, to use the same phrase as the WHO, there is so far “no evidence” (i.e., no conclusive proof!) that they are really reinfections: scmp.com/week-asia/heal… •10/21
But one thing IS clear about these apparent reinfections: there aren't many of them. There are over 800k people who have recovered from Covid-19 so far, most live in areas of high disease activity, and we only have a handful of suspected reinfection cases: … •11/21
… the fact that there are reports of cases of Covid-19 reinfections and not for SARS is just because there are so many more documented cases of Covid-19. See also this thread •12/21
Similarly, the fact that, even though intra-household infection rates are very high, we're not drowned in reports of cases were A infected B who infected A again who infected B again who infected A, etc., in the same household, suggests that immunity is the general rule. •13/21
Now all of this is far from watertight evidence. It certainly doesn't mean that EVERY person who has been infected would be immune. And it certainly doesn't mean immunity would last forever (evidence from SARS-CoV-1 suggests 2–3 years; note the “suggests”). •14/21
So it's certainly not advisable for people who have had Covid-19 (even if they're certain of it, which often isn't the case) to consider themselves permanently safe from reinfection. But I don't think anybody is suggesting this. However, safety isn't all-or-nothing. •15/21
The issue, here, is where the burden of the proof should rest. For a scientific article? Certainly there isn't enough evidence to claim that Covid-19 infection definitely causes protective immunity in humans. Indeed, no such paper has been published (that I know). •16/21
But for making policy decisions at the individual or collective level? Well, that's the thing about decisions: we sometimes have an absence of conclusive evidence either way and we still have to make choices. We have to make do with what we have. •17/21
We should remember, when it comes to “no evidence”, that there is also “no evidence” that Covid-19 doesn't turn everyone who got infected into a brain-eating zombie precisely one year after infection. Nor that people who did NOT get infected won't turn into zombies! •18/21
So the underlying question, which is very much left unanswered, is: “what decision, exactly, do you intend to take on the basis of whether immunity exists (always? or as a general rule? permanently? for a certain time?) after Covid-19 infection?” We should question that. •19/21
But whatever the information is used for, it's important to label its degree of certainty with nuance and accuracy: “little evidence” if you wish, “insufficient evidence”, “inconclusive”, “not an open-and-shut case”, right. But also little evidence to the contrary. •20/21
Sadly, every issue around Covid-19 has become so politicized, with crazy conspiratorialists accusing the WHO (or China, or some government, or whomever) of random evil stuff, that all sense of nuance and scientific accuracy is lost and every statement is loaded. •21/21
Update: the WHO later (yesterday) updated their statement of “no evidence” to the very reasonable “most infected people will get some level of protection, but we don't know how much or for how long”. Much better! •22/(21+1)
