The messenger RNA vaccines and masks

By Larry Corey, M.D.

COVID-19 VACCINE MATTERS: A blog series discussing the evolving science and policy of COVID-19 vaccines, led by internationally renowned experts in vaccine development, Dr. Larry Corey of the Univeristy of Washington, and Dr. Chris Beyrer of Johns Hopkins University.


Key Points:

  • Current phase 3 trials tell us whether COVID-19 vaccines prevent illness and hospitalization, but not whether the vaccines prevent infection and transmission.
  • Despite the excitement around reports of high vaccine efficacy, we will still need to wear masks, physically distance, wash our hands frequently, and avoid large gatherings when a vaccine becomes available until we know more about the ability of the vaccine to stop virus spread.
  • Additional studies will need to be done, perhaps within households and on college campuses, to see whether COVID-19 vaccines prevent virus transmission, allowing us to return to a more normal life.

In the messenger RNA (mRNA) vaccine trials, COVID-19 vaccines are primarily being tested to determine whether they prevent a person from getting sick, from having a prolonged illness, or hospitalization. Importantly, these clinical trials are not focused on whether a vaccine prevents someone from getting the virus at all. In other words, a COVID-19 vaccine may benefit the individual who gets vaccinated, but the virus may still invade the body and it’s possible – if not likely – that a vaccinated person can still spread the virus to others. This is a critical distinction that has received little attention.

If you think about it, infecting someone without them knowing it is a great strategy for a virus whose goal is to “replicate and survive.” HIV is a master of this approach. The HIV virus infects most people while causing minimal symptoms, and persists for years before an individual develops AIDS, and all the while HIV can spread through sexual contact. SARS-CoV-2 differs from HIV in that it’s most infectious in the first few days after acquiring it, and it has explosive infectivity for a few days until the body’s immune response starts to kick in. And, SARS-CoV-2 isn’t transmitted through sex but by less intimate interactions such as talking and sneezing; it’s just way more infectious in a short period of time. Get close – inches – to someone who’s infected and SARS-CoV-2 is highly contagious; a few feet away markedly less. Wear a mask and it’s a heck of a lot less risky.

So, enter a vaccine. What does it do to this dynamic? Answer… we don’t know yet. If I get vaccinated, do I lessen my risk of actually getting infected? Will I walk around with the virus and not know it? Do I still shed the virus from my nose? Is it at the same level as if I were not vaccinated at all? For the same duration? Could I be one of those super-spreaders without me knowing that I am sick? Will we have hundreds of vaccinated people walking around with the virus unknowingly, putting the unvaccinated at an even higher risk of contracting COVID-19? In medical terms: will vaccinated persons be asymptomatic carriers who can still transmit throughout their household, their community, their school? It’s what we call onward transmission.

These questions are of obvious importance. If vaccinated individuals are capable of transmitting infection, then anybody who is not vaccinated fares no differently before than after the introduction of a COVID-19 vaccine. With vaccine hesitancy resulting in fewer people agreeing to be vaccinated, we do not yet know whether and when we will be able to markedly reduce the public health implications of COVID-19 and reduce its circulation in the workplace, in close communities, and stop super-spreading events. With the promise of a vaccine, many continue to ask: will I still have to wear that damn mask? And the answer to this is: until we know more…yes.

The way we generally look at a vaccine preventing infection is whether a vaccinated person makes an antibody and – if they don’t get infected throughout an entire trial – we look at their antibodies pre- and post-vaccination. We have tests that determine whether the placebo groups get asymptomatically infected more than the vaccine groups. For example, the most common respiratory virus vaccine – influenza vaccine – sometimes prevents someone from getting clinical flu but doesn’t often prevent infection. Put differently, one can still acquire the infection even after getting vaccinated. Preventing infection entirely is usually considered to be a higher hurdle for a vaccine.

So, at this point, we do not know – even with the high efficacy reported in reducing symptomatic disease – whether after vaccination individuals are still infectious. Once a vaccine is available, do we need to be careful about whom we invite to our home for dinner? Do we need to continue to wear masks with other people around, especially casual acquaintances or strangers? At the moment, the answer is still yes until we unravel these issues.

Mathematical modelers: vaccination and reduction in transmission

Mathematicians have modeled and made predictions about COVID-19, and they have envisioned all sorts of scenarios. And, one scenario they have created imagines that if we have a vaccine that makes everybody asymptomatic, but individuals still remain infectious, then whoa! We may actually end up with more people who get infected!

Scenarios like these raise the concern that a lot of people will not get vaccinated and they will get infected. We will still see a lot of hospitalizations due to COVID-19. There will still be deaths. This realization helps explain why we must optimize coverage and overcome vaccine hesitancy, especially in persons who are at high risk. Because we certainly are going to see behavioral changes once people get vaccinated. They’ll think, “I got vaccinated, why do I need to wear a mask?” So, those modelers, they’ve forewarned us that we do need to learn about infectivity after vaccination.

In the end, to answer the mask or no mask question, we need to do a study, and that’s where I am today. Discussing the issue and advocating for a study. Households and college campuses, I think, are a good place to consider for this – where we can vaccinate and sample people every other day and see if they acquire COVID-19. And if they do, what’s the amount of virus? Then, we can do some contact tracing to see if people spread it to their roommates and classmates, because colleges are starting to do this anyway. Let’s piggyback on this thing. Try and do a vaccine study and answer the question: mask or no mask? Please tell me – do you think we should do this?

Dr. Larry Corey is an internationally renowned expert in virology, immunology, and vaccine development and a leader of the COVID-19 Prevention Network (CoVPN), which was formed by the National Institute of Allergy and Infectious Diseases at the U.S. National Institutes of Health to respond to the global pandemic. He is a Professor of Medicine and Virology at University of Washington and a Professor in the Vaccine and Infectious Disease Division and past President and Director of Fred Hutchinson Cancer Research Center.