COVID-19 Vaccine Boosters: A Discussion for Pregnant Women

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Back when COVID-19 vaccination first became available to adults a little bit less than a year ago, the decision on which vaccine to get was very simple for most people. You got whichever one was available, because in most cases there was no choice. In the US for example, early on it was often “today we are giving the Pfizer-BioNTech” or “today we have the Moderna vaccine.” Once you had once dose of one, your second dose had to be of the same vaccine. Later on the Johnson and Johnson (Janssen) became available, but very often, in any particular vaccination center there was not a choice. And of course, if you are a child age 5-17, there is just one option in the US: Pfizer-BioNTech. In Europe, similarly, often people had no choice between the AstraZeneca-Oxford vaccine and the Pfizer-BioNTech.

During this passed year, as people were getting vaccinated, here on The Pulse, we also kept you educated with respect to the appropriateness of the COVID-19 vaccines in pregnant women. Thus, you learned that it is absolutely in the best interest of you and your fetus for you to get vaccinated, before or during pregnancy. To drive home this point, I wrote a post debunking a circulating myth that the COVID-19 vaccines harm fertility and cause detachment of the placenta from the uterus. We also discussed issues of women and blood clots in relation to the AstraZeneca/Oxford and Janssen vaccines, bringing home the point that any risk is only very slight and not enough to avoid these vaccines, if these are your only options, which they hardly ever all anyway.

But now, due to increased availability of the vaccines plus the results of studies leading regulatory agencies, such as the US Food and Drug Administration (FDA), to allow mixing and matching between initial vaccine doses and boosters, there actually can be a dilemma for you now, if you go to get a COVID-19 “booster”, which really means another dose — a third dose if you have had any COVID-19 vaccine other than Janssen and a second dose if your first vaccination was Janssen. The dilemma is “which one should you choose”. Very often these days in the US, all three vaccines will be available to somebody scheduled and approved for a booster/additional dose. If you are vaccinated already and now preparing to get your booster, should you get Janssen? Should you get Moderna? Should you get Pfizer-BioNTech?

Although studies are ongoing, really there aren’t enough data yet for the decision to be a no brainer, except, I think for those who so far have received the one dose vaccination of Janssen. Based on what we know, it seems that you should choose one of the mRNA vaccines for your second shot. But there are some data. Let’s take a look at them for some insight.

Studies have been conducted in which antibody levels in the blood have been compared with with different combinations of initial COVID-19 vaccine brand and booster brand. Regarding initial vaccinations, comparing Janssen, Pfizer-BioNTech, and Moderna, Janssen appears to produce the lowest levels of antibodies, but it also seems to produce a better T lymphocyte response, particularly of the T memory cells. This is not what typically is measured on evaluations of immune response, but it is thought that a better T lymphocyte response to equate better retention of immunity over time. Meanwhile, more recently, studies have suggested that antibody levels are boosted better by the mRNA vaccines (Pfizer-BioNTech and Moderna) than by a Janssen booster, regardless of which vaccine the person had as the initial vaccination. This means that if you took the Janssen vaccine as your initial vaccine, it is probably a good idea to choose an mRNA vaccine for your booster; your antibodies will be boosted higher and you already probably have a good T lymphocyte effect from your initial dose.

Now, what if you had an mRNA vaccine for your first two doses? Studies show that the Pfizer-BioNTech and Moderna both produce higher antibody levels in such people than Janssen boosters produce, but there is a dilemma, because, though T lymphocyte effects of the booster are not mentioned in the recent studies, it stands to reason that the T lymphocyte effect will be better from a Janssen booster given to someone who was initially vaccinated with an mRNA vaccine. On the other hand, what prevents symptoms resulting from exposure to the virus is antibody levels, so this weighs in favor of getting a third mRNA shot, until we know more.

As for the difference in effects of the Pfizer-BioNTech versus Moderna as a booster, the recent studies show a little bit stronger boost of antibodies from the Moderna, regardless of what your initial vaccination brand was, but here is another dilemma: In these studies, volunteers received the full dose Moderna (100 micrograms) as the booster, meaning a third regular dose, but if you go to get a booster and ask for Moderna, what they offer is a half dose (50 micrograms). This may sound better to you if you are worried about adverse effects, but importantly, the studies also should minimal adverse effects from boosters for all three vaccine brands. Keep in mind though that the full adult dose of the Pfizer-BioNTech vaccine is only 30 micrograms, meaning lower than the half dose of Moderna. Certainly there are issues related to how much mRNA there is within the lipid nanoparticle carrier particles of each of the mRNA vaccines, so it may not be correct to think of the Pfizer-BioNTech as being less powerful than either the full- or the half dose of Moderna.

Hopefully, when you go for your booster, you’ll get luck and there will be just one option. Take it and be happy that you avoided the dilemma.

David Warmflash
Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.

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