January 18th, 2021

COVID-19 Vaccine Frequently Asked Questions

In case you missed it, over on the New England Journal of Medicine, we now have a list of Covid-19 Frequently Asked Questions.

(Why this NEJM Journal Watch site and the actual New England Journal of Medicine use different capitalization rules for this disease is a mystery. And don’t get me started on the Washington Post, which writes it as “covid-19”– all lower-case.)

It’s been a fun project to put these FAQs together, and my great hope is that it’s useful to clinicians and others who have questions about these amazing vaccines. Turns out I’ll learn a lot too.

So far, the single question that has drawn the most attention is this one:

Do the vaccines prevent transmission of the virus to others?

In my response, I try to highlight the fact that while we don’t have ironclad proof, it is highly likely that they will lower the risk. I urge you to read the full question where I outline the evidence so far.

In my response, however, I wrote this:

If there is an example of a vaccine in widespread clinical use that has this selective effect — prevents disease but not infection — I can’t think of one!

Some colleagues now have pointed out a few examples — diphtheria, meningitis B, and pertussis. My apologies for not mentioning these! We will update the site, and thanks for pointing this out.

Nonetheless, the general (if not ironclad) rule that vaccines typically reduce the risk of transmission to others remains true. After all, this is the primary reason why we have policies on mandatory school immunizations, influenza vaccines for hospital employees, and country-specific entry requirements for the yellow fever vaccine. Rubella immunization in particular is critical to prevent transmission of this otherwise benign infection to pregnant women. We also stress the importance of having immunizations up to date for people living with a person with a weakened immune system for this reason.

Sometimes it turns out to be an ancillary benefit of vaccine recommendations. This happened with expanded childhood immunization for pneumococcal disease, which has lowered the incidence in adults.Thanks, kids!

And it can be a great additional motivating factor for people considering the vaccine who might otherwise be undecided. Getting the shot protects you and protects others.

The challenge for us — as summarized in the New York Times — is communicating optimism about the vaccine’s high likelihood of reducing transmission, while at the same time acknowledging that we don’t yet know how much they’ll do so.

The overall message of the piece is that we’re “underselling” these vaccines. That the proven benefit of preventing severe disease is being undermined by caveats regarding disease transmission — about which we have limited, but promising, evidence.

(I suspect it will be a lot, though not 100% — but we’ll see.)

What to do practically in the meantime is also tricky, especially since the vaccine roll-out will be a process that takes months.

Here I totally agree with this piece’s well-stated conclusion, which I’ll quote in full.

We should immediately be more aggressive about mask-wearing and social distancing because of the new virus variants. We should vaccinate people as rapidly as possible — which will require approving other Covid vaccines when the data justifies it.

People who have received both of their vaccine shots, and have waited until they take effect, will be able to do things that unvaccinated people cannot — like having meals together and hugging their grandchildren. But until the pandemic is defeated, all Americans should wear masks in public, help unvaccinated people stay safe and contribute to a shared national project of saving every possible life.

Perfect! And looking forward to a time when the proportion of unvaccinated people is much smaller than it is today.

18 Responses to “COVID-19 Vaccine Frequently Asked Questions”

  1. Brian Michel says:

    Preventing transmission is an important goal and I agree that there is a high probability that the Pfizer and moderns vaccines will do this. I think this is the main reason why some are arguing for giving more people one injection vs giving the most vulnerable two injections. However, we must remember the reason why “flatten the curve” was the primary goal early in the pandemic. The hospital systems are much more overwhelmed now than they were in April. That is why I favor two injections for the most vulnerable, which should yield more benefit in keeping the icus from being overwhelmed.

  2. Samuel Fink says:

    With regard to article’s conclusion, will you actually be able to hug your young grandchildren? They are not the ones being vaccinated, as the currently available vaccines are available for those 16 and over. I would think we would need to wait for the grandchildren to be vaccinated as well…otherwise, why not hug adults who haven’t been vaccinated as long as we’ve been vaccinated? Thanks for your always on-point commentaries!

  3. Michael Wechsler says:

    What about a patient who had known COVID. What if he were exposed to covid again. Assuming he has sufficient immunity to handle the virus and not get sick, could he be a carrier and pass it on to others?

    • Arturo Azpiroz says:

      After immunization due to having passed the SARS-Cov2 infection, the existence of IgA in the respiratory mucosa is to be expected and not only the IgG expected with the vaccine against COVID-19. It therefore seems that the possibility of transmission should be still lower in the post-covid than in the vaccinated.

  4. Arturo Azpiroz says:

    From my point of view it could be said that we will soon find ourselves in a scenario of “modified Barrington declaration”. With the effective appearance of vaccines for the prevention of severe COVID-19 and their application in populations at risk, the rest of the individuals, whether they can be vaccinated or not, should abandon all preventive measures (masks, safety distance. ..etc), in order to accelerate herd immunity as much as possible and avoid what is already beginning to happen, the race against new strains and I believe that the danger of this is not the virus but the chronification of the unfortunate state of the general population.

  5. Jeff says:


    Do you agree with the NYT article conclusion that those with adequate vaccination do not have to strictly distance and may gather for “meals and hugs”? I’m sure we’ll also be getting this question from out patients soon.

  6. Andi G says:


    As always, thanks for sharing great information. I still have some concern about receiving the vaccine in pregnancy. While I agree, the mechanism of action of the vaccine seems like it would not affect the mother or fetus, what about the chance of fever after receiving the vaccine? There has been some retrospective studies that have seen an increase in autism spectrum disorders with fever in pregnancy. Have you seen any information on this? (This is totally for my benefit but I can’t be the first that has wondered this!) Thanks!

    • Arturo Azpiroz says:

      Regarding the febrile reaction to anti-covid vaccine, we can take as an example what was published in NEJM on adverse reactions to mRNA1273: the possibility of fever after the first dose is irrelevant, however after the second it is approximately 15% .
      The question of the possible or not increased risk of autism due to vaccination during pregnancy (with or without fever) does not seem to have definitive data or they are of doubtful validity. If there appears to be a slight but significant increase when fever coexists with systemic infection in the pregnant woman.

    • Arturo Azpiroz says:

      I am also interested in the opinion of Dr. Sax and I am not pleased to see that this blog page has been “frozen” in only 12 interventions, I think it should give much more controversy and replies.
      Apart from this, I have been looking at the links provided by CC MD without seeing that group of “Suspected COVID-19” which consists of:
      Are clinically compatible + negative PCR ?; Compatible clinic without performing PCR ?. Apart from the possible false negatives, what does COVID-19 refer to without SARS-CoV2?
      It is no longer a question of the efficacy of a vaccine but simply what is all this?

  7. Sam Stampfer says:

    It’s a little alarming that they did challenge studies in non-human primates and yet didn’t specifically test for transmissibility- which would have been easy to do at that time.

    Challenge studies in non-human primates vaccinated with both Pfizer and Moderna vaccines allowed viral replication in the nasal pharynx of non-human primates, yet none bothered to try to isolate live virus from there. Would have been nice to generate that sort of data . . . which they still could try to generate.

    Challenge studies in the Novavax vaccine did not yield any detectable virus by PCR from the nasopharynx, but they used 10^4 PFU of virus in that study versus 10^6 in both the Pfizer and Moderna ones, so they’re not comparable. Not sure how any of these numbers compare to a realistic exposure to an infected person, either.

  8. Roger Kimber, MD says:

    Dr. Sax,

    with regard to your answer about vaccine preventable diseases you were right the first time. There are no viral vaccine preventable diseases where an effective vaccine is available, and the counter examples presented were all bacterial diseases and 2 of the 3, the disease manifestation is toxin and specifically exotoxin caused. So, the counter examples do not apply, and do not invalidate your assertion, IMHO.

    I also heartily agree with Dr. Arturo Azpiroz’s January 19th posts.

  9. Shishir Gokhale says:

    Vaccines protect the individual from disease or at least moderate the severity of disease. Vaccines, specially parenteral vaccines, component vaccines, are not able to prevent acquisition of infection and transmission of the bug as these do not lead to local immunity at the point of entry. Please add to the list of such vaccines IPV. Recent study with modified OPV2 has amply proved it.

    The vaccines against Covid have so far been providing only IgM and IgG antibodies, not IgA antibodies, and therefore are not likely to prevent getting the virus in respiratory tract and its spread. In other words, it will not prevent spread. The only possibility is that the severity, associated morbidity and mortality in the vaccinated people will be less, thereby reducing the viral load, viral excretion, number of people exposed to the virus and chance of spread will be to limited number of people. This is again adding to flattening of curve, not elimination of the virus.

    OPV has been a success in eliminating polio because of local immunity, IPV has not.

    The only hope of eliminating the viral infection and spread is by generating local IgA mediated immune response, possible with a nasal vaccine. All other vaccines can only protect vaccinated individual, not the community.

  10. Philipp Schroeder says:

    Next step ahead: What about long term efficacy/Booster when it comes to the different vaccine approaches related to their working mechanism?

  11. Loretta S says:

    So, “there is no biological reason why receiving the second dose late would diminish the effectiveness of the vaccine, provided it’s received before too long an interval.” My question is: how long is “too long an interval”? I got my first dose and my state did not require the “megacenter” I went to to schedule the second dose. I now find myself in line with over 4 million other people in my state, trying to make an appointment for the second dose through the same system that people use to make their first dose appointment. Needless to say, there are zero appointments available; I checked through the end of June. Now I’m wondering if I’ll have to start over with the first dose when I eventually get an appointment.

  12. Jon Blum says:

    Loretta, there is not enough experience with these vaccines for a definitive answer, but experience with other vaccines suggests that giving the second dose late will work well because of immune memory. In general, ACIP advises against restarting courses in this situation.

    The actual CDC recommendation is at https://www.cdc.gov/vaccines/covid-19/info-by-product/clinical-considerations.html:
    “The second dose should be administered as close to the recommended interval as possible. However, if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration up to 6 weeks (42 days) after the first dose. There are currently limited data on efficacy of mRNA COVID-19 vaccines administered beyond this window. If the second dose is administered beyond these intervals, there is no need to restart the series.”

    The vaccine supply situation is unclear and in flux, as the new administration is basically starting over with the distribution process. I hope it becomes clearer (and better) soon. Meanwhile, consider that the glass is half full: you’ve gotten a dose of the vaccine, and there is evidence that provides significant protection.

  13. Nat Kuhn says:

    Thanks for this FAQ resource. I have not scoured it in detail, but I had a hard time finding an answer to a question many people have: how long should one wait after the second dose to be reasonably confident that you have achieved full efficacy? I see one source saying 7 days for Pfizer, 14 for Moderna. But would love to see an answer in your FAQs (or if it’s there already to see it highlighted more clearly.)

    And yes, perhaps the first time NEJM, but almost certainly the first time XKCD has been referenced in NEJM, another noteworthy milestone.

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HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

Contributing Editor

NEJM Journal Watch
Infectious Diseases

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