COVID Vaccine FAQs

by | Feb 22, 2021


Vaccine production continues to increase and it’s possible that there will be enough doses for any American who wants to be vaccinated to do so by mid-to-late summer. So, let’s dive into common questions about these amazing vaccines (I’m not an infectious disease expert, so each answer is linked to the source).

Should I Prefer One Vaccine over the Other?

Currently, there are two vaccines approved for use in the U.S. under an Emergency Use Authorization: the Pfizer/BioNTech and the Moderna. Two others are expected to be approved soon, one from Johnson & Johnson and another from Novavax.

Dr. Ashish Jha of the Brown University of Public Health says “If you are able to get a vaccine — any vaccine — get it.” Similarly, Dr. Matthew McCarthy, an infectious disease expert at Weill-Cornell Medical Center in NYC said in a recent webinar that the Pfizer and Moderna vaccines should be viewed interchangeably and at his hospital they don’t view the two vaccines to be different in any way.

The Johnson & Johnson vaccine is expected to be granted an Emergency Use Authorization in March. It requires only one dose (as compared to the Pfizer and Moderna which uses two doses spaced weeks apart) but has a lower effective rate (66% vs. 94/95% for Moderna/Pfizer) against symptomatic Covid. However, in its trials, the J&J vaccine was 100% effective in preventing Covid-related hospitalizations and deaths. Source.

How do These mRNA Vaccines Work?

Broadly, all vaccines work the same way — they cause your body to have an immune response specific to a virus so that when your body is infected with the virus your immune system is ready to battle it. Your body creates protective antibodies and T Cells in response to the antigens the virus carries. The specifics of how vaccines create an immune response varies by the underlying technology.

The traditional way to make a vaccine is to introduce dead or weakened virus into the body allowing the development of antibodies without making you sick. This is the way we’ve been making vaccines for over 100 years and it’s proven effective. However, it takes a lot of time and effort to make killed or weakened virus and a lot of testing has to be done to ensure the weakened virus doesn’t cause illness.

The Pfizer and Moderna vaccines work differently; they utilize lab-created bits of genetic material (mRNA) from the the virus. This bit of mRNA “gives instructions for our cells to make a harmless piece of what is called the ‘spike protein.’ The spike protein is found on the surface of the virus that causes COVID-19.” Source. Our immune system recognizes that those bits of spike protein don’t belong in our bodies and generates an immune response (creation of antibodies and T Cells) to fight the virus. Source. A key point is that the spike protein RNA by itself is harmless yet creates an immune response. Amazing.

With these vaccines, the mRNA remains in the cells for just a few days before it is destroyed by the immune system. Source. Again, note that these spike proteins on their own don’t cause Covid-19. Here’s a helpful illustration from the NY Times of the spike proteins on the surface of SARS-CoV-2 that the vaccines target.


The Johnson & Johnson vaccine works similarly, but uses bits of DNA instead of RNA. The Novavax vaccine uses lab-grown coronavirus spike proteins.

How Effective are the Vaccines?

The Moderna and Pfizer vaccines are 94% and 95% effective against symptomatic Covid. This is important: the vaccine trials did not test for whether vaccinated individuals could be infected by the virus. Rather, the “effective rate” refers to whether a vaccinated individual experienced any clinical Covid symptoms.

An important, and often overlooked point, is that even those who develop Covid symptoms after being vaccinated don’t get as sick. Check out this table from Dr. Ashish Jha of Brown University:


So even a vaccine with lower topline efficacy, like the J&J vaccine at 66%, still provides great protection against serious illness. In other words, there’s a lot going on in the data below the headline effectiveness.

When Will I Have Immunity After Getting the Doses?

A common misperception is that once you’re vaccinated you instantly have immunity. The immune response takes time. A study out of Israel of 200,000 people vaccinated with the Pfizer vaccine found that:

  • Between five and 12 days after receiving their first dose, people were just as likely to test positive as were unvaccinated people;
  • At 14 days after the first shot, there was a decrease in positive Covid test results of 33% – 60% (two different study results);
  • Another Israel study found that only 1% of people who had the first shot had enough antibodies to provide immunity within the first seven days.

The second Pfizer dose is recommended at 21 days and the Moderna second dose is at 28 days. For the Pfizer vaccine, the 95% efficacy is at 14 days after the second dose. So, don’t assume you have immunity just because you’ve received the vaccine. Give it some time.

And How Long Will Immunity Last?

I remember when compact florescent lightbulbs were first released. They claimed that they had a lifespan of 20 years. I always wondered how they could tell that if the bulbs hadn’t existed for 20 years. (Of course, it turned out that they didn’t last 20 years.)

A similar problem is at work with immunity against SARS-CoV-2. There hasn’t been enough time since the vaccines were first given to tell how long immunity lasts. A recent study published in Science found that those who have had Covid may have immunity for as long as 8 months. But, the vaccines provide immunity differently than how someone who gets and recovers from Covid has immunity (check out this article for more on this point.) So, we just don’t know. The University of Pennsylvania notes that we know that immunity from the vaccines lasts at least 4 months. Experts don’t think the vaccines will provide permanent immunity and we’ll likely need to get a booster annually.

Once I’m Immune Can I Still Spread the Virus?

This paragraph from the CDC and Infectious Disease Society sums it up nicely:

It is currently unknown whether vaccinated individuals can develop asymptomatic disease and transmit SARS-CoV-2 to others. Therefore, it important to continue practicing other infection prevention measures, such as wearing a face mask and physically distancing. In the Pfizer-BioNTech and Moderna trials, the primary efficacy endpoint of both studies was clinical disease; asymptomatic infection was not reported.

So it is possible that vaccinated individuals can spread the virus. Thus, until more is known it is important for those who have been vaccinated to act as if they are potential spreaders (e.g. socially distance, use masks, etc.). Note that a news report out of Israel yesterday indicates that the vaccines might be highly effective at preventing the spread of the virus. Here’s a link to that story about the non-peer reviewed study: Bloomberg Article. If true, this would be great news.

Will the Vaccines Provide Protection Against the New Variants as the Virus Mutates?

Maybe. Dr. McCarthy of Weill-Cornell is optimistic that the vaccines will provide protection as the virus mutates. Here’s what he has to say about this:

Will COVID19 vaccines work against new viral variants? Mutations are alarming because they can evade antibodies generated by our vaccines. I remain optimistic. Here’s why: Antibodies are just one piece of a complex defense operation. After COVID vaccination, your body starts pumping out antibodies targeted to the coronavirus spike protein. They’re looking for very specific parts of spike. When the virus mutates, it can slip away. Hence, the alarm. But much more is happening inside your body. In fact, there’s another pathway at work. T cells targeting multiple regions of spike are being trained to destroy the virus. As long as some part of spike remains unchanged, T cells should provide protection. Key point: New viral variants may have a major impact on the antibody response, but they’ll probably have only a modest effect on T cells. This is why vaccination is important, even as the virus continues to change. Takeaway: Viral variants may evade one system of defense (antibodies) but it seems unlikely they’ll avoid all of them. The degree to which T cells protect against Covid is not fully understood, but vaccines can provide protection even if coronavirus variants evade antibodies.

Should You Get the Vaccine?


While the vaccine may have some side effects for some people, symptoms are usually mild and dissipate within a day or two. Source. The biggest risk of the vaccine is anaphylaxis, which is a severe allergic reaction. So far, the reported cases of anaphylaxis from the Covid vaccines is about 5 people out of every 1 million (0.0000005%) and there have been zero deaths from anaphylaxis. Source.

Some people don’t want the vaccine because they aren’t worried about dying from Covid so don’t see the need to risk vaccine side effects. But, even if you aren’t worried about dying from Covid, you really don’t want to get Covid because of the risk of nasty long-term effects. According to Dr. McCarthy, you should look at how the virus affects your body long-term. Covid can cause vasculitis, an inflammation of the blood vessels, it can cause blood clots, as well as inflammation of the lungs and heart. The long-term health effects of having Covid are not known. Some people, known as “long-haulers” experience illness from Covid for months and months.

I know people who still don’t have their sense of smell months after having Covid. Think about what is going on in your body that causes you to lose one of your five senses. So, if you weight a non-zero risk of dying from Covid, the possibility that you’ll get really sick, and the risk of having long-term health effects against the mild side-effects of the vaccine it seems clear that getting the vaccine is the way to go. Go back up to the chart provided by Dr. Jha showing how the vaccines have prevented severe illness, hospitalizations and death.

Also – by getting the vaccine you are being part of the solution. The more people vaccinated, the closer we are to herd immunity. The more people vaccinated, the less stress there is on our medical infrastructure and front-line health workers.

Are you concerned that the vaccine was rushed and is unsafe, or that it will change your DNA, or that you are being microchipped, or other concerns? If so, check out this super-helpful page from the State of Missouri, especially the “rumor control” section: Missouri Covid Vaccine Navigator FAQs and this page of Myths & Facts from the CDC.


  1. Sometime in the next few months, the vaccine should be approved for 12-16 kids.

  2. I have a blood cancer and specifically a form of non Hodgkin’s lymphoma known as CLL. It’s my understanding that although I’ve now had both shots the efficacy of the vaccine for people like me with a blood cancer is unknown. Would you please help shed light on this concern I have.

  3. Well that was one of your best IFOD’s, very useful information. I would listen to you over Dr Fauchi. 🙂

  4. Your students are lucky. You did a great job of clearly laying out what is known and being said about the vaccines. Thank you

    • Thanks Leslie!

  5. Excellent coverage of known & unknown facts.


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