Story time: vaccinated people cannot spread Covid19

If you ask me for advice, I say get the vaccine, but do not let your guard down afterwards. Sure, go out more, and re-connect with friends but continue to wear masks and maintain distance in crowded places.

But some of you say, the CDC tells us if one is fully vaccinated, one can treat the pandemic as over. Throw all (pre)caution to the wind.

Let's dissect that recommendation. If you are fully vaccinated, you can still get Covid-19 but they claim you will not get severely sick. They encourage you to return to pre-pandemic living. So they must believe that fully vaccinated people with mild disease cannot spread the virus. They must also believe that fully vaccinated people with asymptomatic infections cannot spread the virus either.

These are important points because half the U.S. population is now fully vaccinated, and if fully vaccinated cases can spread the virus, then potentially lots of people are spreading the virus, invisibly. While we don't get severely ill, the more sick people roam around, the more the virus replicates, and the higher the chance that a rare mutation surfaces that causes another lethal wave of Covid-19.

Starting around April, self-described experts began making claims that "fully vaccinated people cannot spread Covid-19". For example, in this Johns Hopkins piece, an expert said "I think the preponderance of the evidence supports the fact that vaccinated individuals are not able to spread the virus."

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A month or two ago, I gave a talk to University of Denver students, with the key message being "don't judge a research study by its abstract; learn what research methodology it used to draw its conclusions." See this post for more.

At this stage of the pandemic, three research methods dominate Covid-19 research: clinical trials, real-world studies, and lab experiments.

If we want to prove that vaccinated individuals are unable to spread the virus, what kind of study do we need?

We can immediately rule out the lab experiment. Testing blood samples for antibodies will never yield data on clinical outcomes such as symptoms, infections, transmission or spread.

Spread is a very hard outcome to assess. You need to first identify infected people, and then do contact tracing to figure out the transmission chains.

Can clinical trials yield such data? Absolute not, if we use the vaccine trials as they were designed. Those trials only tested participants for infection once every month or two, and most cases were self-reported, meaning that they only identified a fraction of the infections. Moreover, no contact tracing was attempted so we have no idea who transmitted the virus to whom. Therefore, these trials yielded no data about spread.

What about real-world studies? While we have big sample sizes for these studies, they also contain nothing about transmission chains. The aforementioned Johns Hopkins article cites this CDC study. I fail to see how the data proved that vaccinated people cannot spread the virus. The paper shows that vaccinated people are less likely to be infected (on this, I have concerns about inadequate debiasing of the data), and then they stretch that result to claim that the vaccine stops transmission.

Nevertheless, the study made no attempt to figure out who transmitted the virus to whom. For example, how do we know that one of the vaccinated and Covid-positive participants did not infect a dozen unvaccinated co-workers? I don't know the answer, nor do I think that is the most likely scenario but I fail to find any evidence supporting the claim that vaccinated individuals do not spread Covid19 in that study.

So what we have is "story time". Readers of the study are lulled to sleep by the nice dataset that was collected to measure overall vaccine effectiveness, and as they doze off, the researchers feed them the story of preventing spread, which has little if anything to do with the data.

 

P.S. [7/14/2021] In addition to story time, we have contradictionary evidence that gets ignored or dismissed. For example, the Las Vegas Review Journal just reported that 10 vaccinated health-care workers got Covid-19, possibly all during a pool party. Eight of these are fully vaccinated. Only 1 unvaccinated attendee got sick. So 10 out of 11 cases from that party were vaccinated. 

They only know about these cases because the health care workers decided to get tested. There are probably more cases because many attendees may not have gotten tested. Note, however, that fully vaccinated people are more likely to ignore mild symptoms and not get tested.

If fully vaccinated, PCR-positive people cannot spread Covid19, each of these 10 healthcare workers were infected by an unvaccinated, infectious person or persons. Even in this small example, we don't know who spread the virus to whom.

Regrettably, the Review-Journal printed a fallacy that I have pointed out from day the first clinical trial result was published. It said "In laboratory tests, the Pfizer and Moderna vaccines were about 95 percent effective in preventing symptomatic cases of COVID-19. That means that about 1 in 20 people could become sick after being vaccinated, though their infections would be less likely to be severe." In these trials, the case rate in the placebo group - people who got saline shots - was only around 1 percent. If their interpretation were correct, then the vaccine bumped the chance of getting infected from 1 percent to 5 percent - 5 times worse! See this post for how to interpret vaccine efficacy.

Comments

[Michael Droy]

The question is not whether we are 80% safer or 95% safer.
The question is whether it was a big risk to start with, or that lockdowns and mask wearing were ever valid policies.

It wasn't, they weren't.
never mind the economy, closing health services for almost a year will shorten lives by many more years than Covid has.

[Kaiser]

MD: Asia, and even the UK, outcomes show otherwise.

[Ken]

I did read somewhere that the epidemic modelling showed that for infected vaccinated subjects transmission was definitely not zero, but less than for infected non-vaccinated subjects. I think this may be something that they will attempt to extract from the Israeli data, as one of Pfizers aims was to collect real-world data. This is going to be important as we move to a mainly vaccinated population, as it will determine the level of vaccination required to end the epidemic. As to it not being a big risk, the 3% of 30-50 year olds in Australia with COVID that spent time in intensive care would disagree.