Does vaccination against COVID-19 limit transmission of the virus? It would seem to make sense that if the vaccines prevent people from getting sick from the virus, they should reduce transmission of the virus. However; in science and medicine, we need to confirm what makes sense with information obtained through scientific methods.
The randomized placebo-controlled trials conducted back in 2020 were not designed to answer the question about transmission. These studies had to be done with the intent of making sure the vaccines were safe and worked well enough to be given to the general population.
The studies showed overwhelmingly that the vaccines were extremely safe and very effective at keeping people from getting sick. The most certain way to determine if the vaccines are effective at preventing transmission would be to conduct multiple large, randomized placebo-controlled trials. This would be unethical. Now that we know the vaccines protect people from getting severe illness, hospitalization, and death; to have a study where we withhold the vaccine from a set of study participants would simply be wrong.
While we will never have a randomized placebo-controlled trial of the effectiveness of the vaccines against transmission; that does not mean we don’t have other ways of answering this question. There are numerous studies and analyses that confirm what is biologically plausible - the vaccines have a significant but limited effect on transmission. In the supplemental information I will include links to some of these studies. In this article I will briefly summarize these studies.
Researchers have tried to indirectly estimate transmission by looking at the “viral load” in vaccinated versus unvaccinated individuals. In general, it can be said that these studies show that vaccinated individuals have a viral load comparable initially to unvaccinated individuals but the levels drop off much quicker in the vaccinated. These studies have been questioned since it is possible that some of the viral particles quantified in the samples may be “dead” (inactivated or unable to infect other cells). Other studies have taken samples then cultured them to see if there is virus that is still able to replicate - but even this does not truly show if someone is more or less likely to pass on the virus to others. These studies have overall suggested significant but limited overall effects of vaccination against transmission.
In the absence of randomized trials and the limitations of what I will call “benchtop test tube experiments” - we need to look at well done observational studies of people in the real world.
In one analysis researchers took advantage of Danish national data comprising 24,693 households with a primary case of SARS-CoV-2 infection and 53,584 household contacts. The researchers looked at infections occurring in individuals and observed how often they passed on the virus to household contacts. People in this study that were vaccinated; but still got the virus, were significantly less likely to make their household contacts sick than those who were not vaccinated prior to getting COVID-19. The researchers looked at all combinations of vaccinated versus unvaccinated primary infections and vaccinated and unvaccinated household contacts. The data can be viewed in the article linked in the supplemental information - in summary vaccination significantly impacted both transmission and susceptibility to infection no matter the combination of primary and secondary cases.
Another study looked at data from hundreds of thousands of people living in the UK. This study published in The New England Journal of Medicine showed that when an unvaccinated individual brought the virus into their home, they were 40-50% more likely to pass on the virus to those living in their house than those who were vaccinated.
Finnish researchers studied household contacts of those infected, finding a significant reduction in transmission from the person infected to the other adult(s) living in the household if the person infected was vaccinated. There was a lower number of children that became infected from the vaccinated infected adults than when the primary adult case was unvaccinated, but the finding was not statistically significant. It should be noted that in Finland, elementary schools were open for in person learning during the time of this study. This could have confounded the results as these kids had lots of outside of the home exposure opportunities. It is also notable that elementary school aged kids were not able to be vaccinated during the time of the study.
Studies in Israel have examined transmission showing significant benefit from vaccination. These studies were published in the American Journal of Epidemiology and the journal Science. The details are available through links in the supplement information.
An interesting study conducted in the US involved prison inmates. During a time when the Omicron variant was predominant, about 22,000 cases of COVID-19 within 35 prisons in the United States were followed. It was found that those who had at least 1 vaccination were (at least) 24% less likely to pass on the virus. One major factor in how effective vaccination was in preventing transmission was time from vaccination. Another major factor was just how many times the primary case was vaccinated. In other words, being boosted recently looks like it has a substantial effect on transmission and this statement applies to those infected with an Omicron variant.
When thinking about how our immune system works - these findings are NOT surprising. Vaccination should reduce transmission, but the effects may lessen with time therefore booster shots should reduce transmission in those how have had a primary vaccination.
As an introduction to how the immune system works against the virus read the article “Immune System vs SARS-CoV-2”. The immune system is a complex and fascinating system; I encourage you to learn more. I will include in the supplemental information to that article, links to other learning opportunities.
Go to vaccines.gov to find a location to get vaccinated.
All statements in this article should be attributed to myself, not any organization I represent or my employer. This article presents a medical perspective - not a statement intended to be for or against any public policy or politician.
Russ Coash, PA-C
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