Risk Analysis of the Moderna and Pfizer Vaccines, part 3

What is the risk I will pass on Covid even if I get vaccinated?

At first this seems as if it should be an easy answer: it would be the risk of passing on Covid if you get it, multiplied by the risk of the vaccine not protecting you from getting Covid (5%).  But now we can see there are two complications with that simple formula.

Why this is a hard question

First, even if the vaccine doesn't protect you from Covid entirely, it might reduce the severity of the infection and therefore reduce the amount of viral shedding.  Given how few people actually got Covid who were given the vaccines, there is simply not yet enough data to prove that the vaccine reduces the severity of the disease if you get it.  There are some early indications that this might be the case and it is very reasonable to think it might be the case, but it is not yet medically proven.

Second and more worrying, however, is that we don't know for sure that if the vaccine keeps you from getting Covid (the disease) it also keeps you from spreading the disease.  The way they tracked people in the trials for both vaccines, is that they tracked all symptoms that people had, and then verified the presence of the virus in people who reported the right symptoms.  This leaves open the possibility that many vaccinated people caught the virus, that the vaccine made them asymptomatic, but that they were still contagious even though they showed no symptoms.

It should be noted up front that this is a *possibility*. Some people have erroneously heard about this possibility and concluded that we know you can still transmit the disease even after being vaccinated.  We don't know that this is the case; in fact, it could well be that the vaccination as effective at preventing the transmission of the disease as it is at preventing its symptoms.  I believe this to be the more likely scenario, in fact.  But this is not proven one way or another yet.

How to estimate an answer

This being the case, we have to come up with a way of getting a reasonable estimate of the likelihoods here in order to build a risk assessment.

I think the most useful study to use here is this one (https://www.nature.com/articles/s41591-020-0869-5), which looks at the percentage of transmission that occurs before onset of symptoms: 

The useful thing about this study is that they looked at things from the perspective of timing of the viral load, but they also cross-checked with studies that looked at transmission in countries where extremely careful contact tracing was happening and which were deducing from that data how much spread was happening before symptom onset.  This two very different ways of looking at things came up with very similar answers: somewhere around 40ish to 50 percent of transmission happens before symptom onset--say 45% to take an even number.  This answer would include both asymptomatic and pre-symptomatic transmissions, which means that an absolute minimum of 50% of transmission happens in conjunction with some symptoms--more, almost certainly, because this number includes transmission that happened just before symptoms started.  Since the vaccines are 95% effective in preventing the disease to the extent where no symptoms ever show up, we can say that it's likely at least 49.5% (say, 50% rounding off) effective at preventing transmission.

The next question is, what percentage of that ~45% of transmission that is happening before symptom onset is truly asymptomatic and what percentage is pre-symptomatic?  I have been unable to find really convincing answers here, but the sense I get from the research is that this split is heavily weighted towards the pre-symptomatic--say, 80/20 in its favor.  So I would personally bump up the likelihood that the vaccines protects from passing on the infection from 50% to about 65% based on this split.

But it gets better than this.  Recall that what is happening at least some of the time with asymptomatic individuals is that their immune system is fighting off the disease by producing antibodies, just not by also ramping up on the other general disease-fighting mechanisms which create all of those typical illness symptoms: fever, congestion, aches, etc.  

While the vaccine trials did not do routine testing for the virus in all of its participants, they did do testing on the antibody production which the vaccine generates, and we do know that it causes antibody production in its recipients equal or greater than we see in those who have cleared the virus from their systems naturally.

Recipients of the vaccine are therefore in a situation after having been vaccinated that most of the asymptomatic infected people are not in until after they have cleared the disease.  What this means is that however much asymptomatic transmission happens because of this time period before antibodies are generated, that much transmission will also be blocked by the vaccine.

Conclusion

Therefore I think it is reasonable to bump up the percentage likelihood that the vaccine will prevent you from transmitting the disease even further--let's say, to 80%.  Personally, I think this is conservative and that the true number will be closer to 95%, but I'm fine with using the 80% figure for risk estimation.

(EDIT 1/11/2021) Update

There was some important data I missed for this section of the series.  There is actually some data from the Moderna trial specifically which gives a good early indication that we are seeing that protection from transmission from the vaccine.  This link: https://twitter.com/EricTopol/status/1338872330538237955 points out a part of the FDA briefing document that I had missed, which indicates approximately a 75% reduction in transmission of the virus after a single dose of the Moderna vaccine.

I think this is very encouraging because this would include a time range during which the vaccine doesn't have much effect at all (the first couple of days), and a longer time period during which your body is ramping up its immune response.

So if just the first dose confers a 75% reduction in asymptomatic spread, I would expect the full two doses to be closer to my hoped-for 90-95% effectiveness.  80% is still a good conservative estimate to use, so this data doesn't change my bottom line; it just makes it more reliable.