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.