The Centers for Disease Control (CDC) recently published their updated recommendations for precautions by people who are fully vaccinated against COVID-19. They needed to review available data on the efficacy of the available vaccines in the US – Pfizer and Moderna; J&J is just coming out. Their recommendations are welcome news to those who are fully vaccinated, which means you have received your second dose of vaccine more than two weeks ago (it take up to two weeks for the antibodies to fully form).

Their recommendations are, in non-health care settings, and for those who are not at high risk:

  • Visit with other fully vaccinated people indoors without wearing masks or physical distancing
  • Visit with unvaccinated people from a single household who are at low risk for severe COVID-19 disease indoors without wearing masks or physical distancing
  • Refrain from quarantine and testing following a known exposure if asymptomatic

Hopefully this will be further incentive for those on the fence to get vaccinated. This is a concrete manifestation of the ability of the vaccines to return life to something close to pre-pandemic normal. But – what took the CDC so long to make these recommendations? Could these not have been made once the vaccines were given emergency use authorization based on evidence of safety and efficacy? The delay is mostly because the evidence of efficacy only looked at the prevention of illness, not the prevention of being an asymptomatic carrier of the virus. Theoretically, someone who is fully vaccinated and protected about 95% from getting sick with COVID-19 could still catch the virus and pass it on to someone else, without even having any symptoms. The CDC wanted independent evidence that the vaccines also prevent spread.

What is the a-prior likelihood that the vaccines reduce spread? Pretty high. These vaccines work by provoking an antibody response that will attack the virus before it has a chance to produce a serious infection. Efficacy is therefore tied to reduced viral load, which in turn will affect how much virus is shed by infected individuals, and therefore how contagious they are. The real question is – how much do the vaccines reduce spread? It would be surprising if they didn’t reduce spread at all. But it is theoretically possible that they do not reduce spread sufficiently to change pandemic recommendations for mask wearing and social distancing.

Studies for the H1N1 flu vaccine, for example, show that they reduce shedding of virus and delay but do not completely eliminate transmission to unvaccinated subjects.

How does this impact herd immunity? The more the vaccines prevent spread, the easier it will be to achieve herd immunity, the point at which the virus can no longer spread through a population because there are not enough susceptible hosts. Vaccines do not need to completely prevent spread to produce herd immunity, but this will affect the percentage of the population that needs to be vaccinated before herd immunity is achieved.

With these unknowns, the CDC was being cautious in keeping full pandemic precautions in place until we had sufficient data on the effects of the vaccines in the real world. They also were waiting for the total new infections to drop significantly before backing off on precautions. What is the current evidence for the ability of the COVID-19 vaccines to prevent spread?

In the original studies used to show efficacy of the Moderna vaccine, they also gathered secondary data on the prevention of asymptomatic infections. They found:

Amongst baseline negative participants, 14 in the vaccine group and 38 in the placebo group had evidence of SARS-CoV-2 infection at the second dose without evidence of COVID-19 symptoms. There were approximately 2/3 fewer swabs that were positive in the vaccine group as compared to the placebo group at the pre-dose 2 timepoint, suggesting that some asymptomatic infections start to be prevented after the first dose.

That is a 2/3 reduction in positive nasal swabs for SARS-CoV-2 after the first dose, which means the protection is likely to be much higher after the second dose. This is solid evidence that the Moderna vaccine reduces the risk not only of the disease but asymptomatic infection, and therefore spread.

There are also two studies of the Pfizer/BioNTech vaccine after release. In a UK study involving hospital workers they did not look at reported symptomatic disease but data from nasal swabs using a PCR test for viral infection.

A single dose of BNT162b2 vaccine demonstrated vaccine effectiveness of 72% (95% CI 58-86) 21 days after first dose and 86% (95% CI 76-97) seven days after two doses in the antibody negative cohort.

Therefore that 86% reduction after the second dose includes a reduction in asymptomatic cases, and therefore this is a reduction in the total number of people who can spread the virus, not just those who report symptoms. There is also an Israeli study, reported but not yet published, which found:

Meanwhile, a press released on a pre-published, not-yet-peer-reviewed paper from the Israeli Health Ministry and Pfizer found that the vaccine appeared to reduce all coronavirus infections—including asymptomatic infections—by 89.4% and symptomatic infections by 93.7%.

This same study found that viral loads decreased after vaccine doses. None of this is surprising – the vaccines are working as predicted by their mechanism of action. Again, the only question was a matter of degree. We now have solid but preliminary evidence that the first two vaccines released in the US not only prevent disease, they also significantly prevent spread. But of course, researchers want more data. This is a critical question, and will guide future recommendations for pandemic prevention and our understanding of what it will take to achieve herd immunity.

As has been said throughout this pandemic – we are building the plane as we are flying it. This is a novel coronavirus, these are new vaccines, and we are learning as we are going. Gathering data in the world (not just the lab) in real time is critical, but it takes time to gather and analyze data. Meanwhile we need recommendations. It is understandable, then, that the CDC erred on the side of caution in relaxing their recommendations for the fully vaccinated, but the data now supports them. They will also continue to monitor the pandemic and evidence of vaccine efficacy in preventing spread and update their recommendations accordingly.

There is reason for optimism, however. The vaccine are extremely effective in preventing death, hospitalization, and symptomatic illness. They now also appear to be effective in preventing asymptomatic spread, but certainly more data will be useful. This bodes well for the potential of the vaccines to achieve herd immunity, as long as there is sufficient vaccine uptake. In a way, the scientists have done their job and now it is up to the public to do theirs.

We are also in a race against time to get this pandemic under control before new variants emerge that can evade existing vaccines. There is a real risk that COVID-19 might become endemic in the human population, or that we will need periodic vaccine updates to cover new strains. We may have a window of opportunity to prevent that if we get as many people vaccinated as quickly as possible, and maintain pandemic precautions until it is truly safe to relax them.


Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.