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Tuesday, April 27, 2021

Asymptomatic and presymptomatic spread of SARS-CoV-2

It is widely believed that a substantial amount of viral spread is due to individuals who are transmitting the virus but have no symptoms (asymptomatic spread) but there's so much misinformation about COVID-19 out there that I'm having trouble sorting out real science from fake science so I've become skeptical of just about everything.

I'm not talking about the kind of fake science being spread on FOX News, I'm also talking about misinformation spread by ordinary people like me and the typical readers of this blog. We might do it inadvertantly but it's still wrong.

What's the real data on asymptomatic spread? I don't know, but here's a summary of the issue in a recent issue of Science. It sounds good to me because the authors take steps to address questions that seem obvious.

Rasmussen, A.L. and Popescu, S. V. (2021) SARS-CoV-2 transmission without symptoms. Science 371: 1204-1207. [doi: 10.1126/science.abf9569]

The first thing we need to understand is that there's a difference between asymptomatic spread and presymptomatic spread. Presymptomatic spread occurs when somebody is infectious but the don't know they've got COVID-19 because the symptoms haven't appeared yet. As the reviewers point out, both are important but it's difficult to sort them out. At the risk of over-simplifying, I'm just going to give some of the highlights from this review. You'll have to read it yourself to get the nuances.

Asymptomatic infections probably represent about 20% of all cases. Are these people asymptomatic because they have a milder infection, in which case they are less likely to transmit the virus? Do they shed fewer virus particles and/or shed them for a shorter period of time? The answer appears to be "yes"; asymptomatic patients are less likely to shed the virus. You can see that in the figure above where the solid orange line (asymptomatic shedding) is lower than the solid green line (presymptomatic shedding).

But the really interesting part of this analysis is the data on when a patient is infectious. As the graph shows, immediately upon infection you enter an eclipse phase of a day or two before you start to produce new virions and become infectious. The infectious phase follows immediately after that and you are most infectious long before you notice any symptoms. Symptoms typically don't appear until 3-7 days after exposure.

Unlike SARS-CoV, SARS-CoV-2 viral loads are highest at symptom onset and up to a week after, which suggests substantial presymptomatic shedding. Therefore, people are likely contagious for a relatively long period and when they are unaware they have been infected or exposed.

It looks like most of the transmission occurs from the 80% who will develop symptoms but who are spreading the virus before they know they are infected. That doesn't mean that symptomatic spread is irrelevant.

Although asymptomatic individuals appear to be contagious for a shorter period of time and may pose a lower transmission risk, they still pose a substantial public health risk as they are more likely to be out in the community.

Whenever you are discussing the pandemic and looking for solutions it's important to keep in mind that it's people who don't have symptoms who are responsible for most infections. The only way to stop this is for everyone to act like they are potential carriers and wear a mask and avoid social contacts. Other things might help a bit but we need to keep our focus on those measures as the best way to save lives in the short term. Everything else is at best a distraction and at worst counter-productive.

Vaccinations will help in the long run but they're not going to save many lives in the next few weeks. If you are vaccinated then you can either flaunt it by taking off your mask and eating at outdoor patios with your privileged friends or express your solidarity with the unvaccinated by wearing a mask and avoiding social contacts just as those less fortunate than you have to do.


  1. I am fully vaccinated, but I still wear a mask when out and about around people so that I don't look like one of those #$$#@ anti-maskers. There is also that ~ 5% chance of break-through infection where I could still get the virus.

    1. I agree with your decision. It's interesting that there seems to be a significant difference in attitude between Canadians and Americans. (I don't know about other countries.)

      Our government in Canada is telling its citizens (in TV ads) to maintain social distancing and wear a mask even if you're vaccinated because we're all in this together and we won't beat the pandemic until we achieve herd immunity. It's an argument based on social responsibility and solidarity.

      The American government is telling its citizens that you can be released from the "burden" of social responsibility as soon as you are fully vaccinated. The American view seems to be that individual rights have been restricted during the pandemic but individuals can be rewarded by getting them back as soon as they are vaccinated. It's an argument based on the rights of the individual taking precedence over collective behavior.

      We always knew about this difference - socialized medicine is another example - but it's quite interesting to see it play out in real time. Our Canadian scientists are saying that you should maintain social distancing after getting vaccinated while American scientists are saying the opposite. It shows you that the correct "science" isn't as obvious as you think.

  2. I worked in COVID testing lab for a few months early in the pandemic, and from the anecdotal evidence I saw there are definitely asymptomatic carriers and spreaders. We did surveillance testing of selected groups, and we found individuals who were asymptomatic but had viral titers the same as or even higher than people who had symptoms. You can't get very accurate viral titers from the NP swabs we were using, but we were seeing 10 fold higher titers (by RT-PCR) in some asymptomatic carriers compared to sick individuals.

  3. I think you're not quite right about the majority of Americans and the current federal government position. The current federal government position is meant to encourage progress toward herd immunity. There is unfortunately a dedicated minority that is likely large enough to prevent herd immunity from happening, which is a subject of some despair and outrage among the majority.