I live in the US. I'm 37 years old and fairly healthy though not in perfect health with regard to covid risk (I'm a little overweight and have a blood clot in my personal history). I've never gotten covid as far as I know. I got my second dose of Pfizer on April 21, 2021. I am tentatively planning to get a booster shot soon, but I'm not certain whether I should or not.  When should I get my booster, which vaccine should I get for my booster, and why? [Edit 11/19/21: and should I get a booster at all?] 

Considerations are as follows. A lot of the following comes from the comments and responses to this question about booster effectiveness and in particular the comments by Elizabeth. This other question about which booster shot to get is also relevant.

Getting any booster would almost certainly increase my immune defense to covid substantial and have no long-term or major direct side effects. Pfizer and Moderna boosters are approximately functionally equivalent. A J&J booster might be slightly worse or slightly better.

But getting a booster now might have indirect long-term side effects. In particular, Elizabeth suggested that there could be a limit to the number of mRNA shots that one can get in a lifetime, either against Covid or against all diseases in the aggregate, because the body could develop immune reaction to mRNA vaccines, or side effects could increase with the number of doses. If this is true, then it might make sense to wait on a booster, until a more effective booster comes out, for instance one specifically tailored to the delta variant. Or to wait until the timing of the booster is maximally efficient to improve immune function.

Also, there may be legal or administrative restrictions on getting more than one booster, so again it may make sense to wait until a better booster is available. Even if I were willing to lie to get around these restrictions, such lies might prevent me from having an accurate vaccination card, which could cause problems down the line.

And I've also seen the theory that mixing and matching vaccines may be more effective than getting all the same vaccine, because it would promote a more diverse immune response. But that the three vaccines available in the US are basically equivalent in this regard. So it might make sense to wait until another vaccine is available in the US or even travel to another country to get another vaccine (not a trip solely for that purpose, but I enjoy travel anyway and it would be a nice excuse to do more travel).

My inclination is that I'm probably overthinking this and am biased towards inaction because I don't relish putting up with a day or so of vaccine side effects. It's probably best to get a booster as soon as reasonably possible, and I should probably just get a Pfizer booster to match my initial Pfizer vaccination as I believe that's the current recommendation of the medical establishment. The immediate benefit of greater immunity now outweighs the concerns above, especially given that I have been returning to a more normal life (traveling, going to bars and restaurants, riding public transit, etc.) and therefore the extra immunity is immediately quite relevant. And we're unlikely to have much better evidence on the questions above anytime soon, nor are we likely to have a booster specifically for delta or other new variants anytime soon. 

But I'm not sure, a part of me is tempted to wait because I feel like I'm pretty well protected with two shots of Pfizer, and I would be giving up an unknown and possibly quite large amount of future utility by getting a booster now. For instance, I could be using up my lifetime ability to take mRNA vaccines, and there could be a future mRNA vaccine against a much deadlier new future pandemic or against HIV or against cancer or against a much deadlier strain of Covid. [edit 11/27/21 -- original antigenic sin makes this last possibility (vaccine against a deadlier strain of covid) less relevant. Thank you, evhub for this comment pointing that out.]

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I'm in basically the same boat as you, 30ish adult, Pfizer regime completed in April. I too have been lazy and uninterested in side effects (knocked out for a day after second dose). And I too am exceptionally interested in hearing discussion of "should I" as well as timing/choice thoughts. So -- just throwing out my thoughts. (Caveats: I'm an econ/stat type, my last bio-ish class was in high school and I remember none of it).

Should we get boosters?

I think my basic answer is "yes". The longer answer is:

  • if it will allow you to reduce other precautions: yes. Free, very low risk of complications and reduces risk of covid -- allowing precautions to relax.
  • if you got covid, and have been vaxxed: don't bother.
  • if you don't/can't risk compensate (have already relaxed all precautions, or will never), haven't had covid: yes, but YMMV in terms of benefits.

I haven't given any consideration to "lifetime mRNA vax limits" until your mention, I have no idea how plausible that is -- but my current belief is "20% chance that's a real issue down the line".

So take everything below as being "conditional on getting a booster at all."


I plan to try to get a booster in a few weeks. The holidays are subsequently anything in January/feb are likely to be high risk for everyone. It doesn't seem unlikely to me that in terms of picomorts or QALY risk (etc) this winter -- where we still face reasonable case counts, but effective treatment regimes are still being rolled out and assessed (for now) -- is probably the highest risk period (for covid-19) remaining. Waiting until after winter thus makes little sense to me. If it going to happen in the next 6 months, it should probably be in the next month.


I have a mild preference for a Moderna booster over a Pfizer booster. Moderna efficacy seems to have been consistently a smidge higher for everything -- likely because its a larger dose (ETA: looks like booster dose is smaller so this factor may be ignorable). This + the benefits of some variation in vaccine regime makes it seem preferable. Two points on this however:

  • This is a small risk change -- not worth it if it forces you to change plans or to do something "risky" without a booster. Given the choice I would choose Moderna -- I'm not sure its worth much to search it out.
  • These benefits come with some risks. I've had two doses of Pfizer, and no reactions that warranted hospitalization or the like. No guarantees here -- but switching it up does add some risk. (so does just another round of pfizer).
  • "Larger moderna dose" may be a negative if you are worried about some "lifetime limit on mRNA doses".



Would genuinely like to hear more debate on "should", "timing", and "choice". "Should" seems likely it will be well covered by media shortly. But "timing" and "choice" are things I doubt I'll see outside of this venue.

This prison study is weird but appears to show that Moderna is MUCH more effective than the other vaccines. Applying Robin Hanson's heuristic that the non-headline numbers in a study are less biased than the headline numbers, we should maybe treat this as more credible than official estimates of relative efficacy.

Does it really show that? Looks hoplessly confounded to me:
This sounds like serious confounding. In all likelihood, prison docs had X doses and triaged, and then repeated. I will say though -- I do still think Moderna primary was likely better, but I've (since writing) found out that the booster is half the dose of the primary dose -- so I doubt Moderna's increased efficacy translates to the booster.
On somewhat further investigation (really limited here -- lets not lean too much on it) -- the Moderna boosters are half the dose of the Moderna primaries. If you believe, as I do, that the primary reason for increased Moderna efficacy in trials was due to dosing, then the reduced booster dose means that the reason I give above (higher efficacy) is no longer a relevant factor. I do think @npostavs is right that this study is likely quite confounded. Though I do still believe Moderna primary doses had reasonably better efficacy against OG covid, and in all likelihood against delta and now omicron.

Adding in post: Omicron both increases short term value of booster and may increase risk of hitting any lifetime cap (its further evidence for more future variants that will warrant boosting)

Why should we think a lifetime limit exists?
Based on this argument. It's not especially strong but is enough to make me want to research further.

I'm still uncertain myself. My key crux at the moment is how side effects of the 2nd dose compare to side-effects of the 3rd dose.

I remember some posts that suggest the amount of people who sought medical help after the 3rd dose was concerningly higher.

While researching I found https://www.cdc.gov/mmwr/volumes/70/wr/mm7039e4.htm?s_cid=mm7039e4_w as a source and going from 0/0->2.1/1.4 (Moderna/Pfizer) indeed sounds concerning. However it seems like the 0's were only based on a total of 64/66 patients so a good part of that difference might be random noise. 

Going from 25.0/15.2 to 35.2/18.1 (Moderna/Pfizer) for "Unable to perform normal daily activities" seems like it's a concerning increase in side-effects for Moderna but not for Pfizer. 

If there's immunity against PEG build up that produces the side-effects and potentially creating something like a life-time-limit for mRNA vaccines we should see that by increased side effects.

It's been a while since the above publication so if anyone finds more data it would be great to look at it.

For what it's worth, the CDC reported that side effects to the booster are less compared with the second dose, and that was also my personal experience.

This would be really good  news for me, since my second shot effects were severe. I don't trust the CDC on this so went looking for papers and found this pre-print from Israel, that found booster symptoms were in the same ballpark as second shots. This release from the CDC found the same thing.
It seems strange to link to a statement that's not by the CDC if you want to talk about what the CDC reported. In general I'd love to see data.
I was linking to a news article on CNBC that quoted the CDC director. in the third paragraph. I didn't take the time to track it back to the CDC directly.

I have decided to get my booster of the Pfizer vaccine and am scheduled to get it on Monday, November 29, the soonest day that was acceptably convenient with regard to my concerns about side effects the next day.

Reasons for the decision were as follows. I'm fairly confident that I'm making the correct decision to get a booster very soon. I'm less confident as to whether I'm choosing the best booster, but I don't think that's likely to make a big difference.


Decision to get a booster very soon

The new Omicron variant increases my risk from covid over the time frame of the next few months. I want my booster to have a substantial effect before Omicron becomes highly prevalent in the US. This was my main motivation to act very soon. I had been tentatively planning to get a booster within the next week, but may have put it off for longer before learning about Omicron.

I learned about original antigenic sin, which makes the concerns about waiting for a vaccine specific to new variants less relevant.

The CDC reported yesterday that covid boosters have less side effects than the second shot, which reduces the credence of the hypothesis of a lifetime limit on mRNA vaccines and also reduces my short-term concern about dealing with the side effects of the booster.

Decision as to which booster to get

Since I now want to get a booster very soon, the possibility of traveling to get a booster not available in the US is no longer appealing. And the risks of traveling are greater due to Omicron as well.

Convenience -- the first place where I went to book a booster (Walgreen's near my house) only allowed me to get a booster of Pfizer since my original vaccine was Pfizer. (In reality, this may have been the biggest reason.)

Getting a booster of the same vaccine as the original is the generally accepted establishment medical advice (based on Walgreen's requirement and also my doctor's advice).

The study referenced in this comment shows weak evidence that a Pfizer or Moderna booster after first two shots of Pfizer is more effective than a J&J booster. But Moderna might be the most effective of all.

Getting the same vaccine as I have already gotten in the past is dealing with something that's more of a known quantity to my body. It seems less likely that I would have unusual side effects with the same vaccine as compared with a new vaccine that I haven't tried yet.

Getting a booster of the same vaccine puts me in a more widely studied cohort, as noted here which might means I'll have more information relevant to my situation going forward than if I boosted with a different vaccine.

I have some thoughts that it might be better to get a J&J booster anyway since it is less likely to hit the lifetime maximum mRNA issue, or because it might do better in stimulating other areas of the immune system besides antibody production, but that is balanced out by the reasons above.

After a bit of research, I did not find any articles about a lifetime limit on mRNA vaccines. [Edit 1 -- found an article. See section But it's an expert opinion submitted in connection with a lawsuit] [Edit 2 and the authors are affiliated with an anti-covid-vaccination group so has a stronger likelihood of bias as compared with a journal article. So it is likely quite biased.]

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Why don't you phone around GPs and ask to find one who will give you AZ? My brother is 29 and in Perth and was able to get AZ in early July that way. I'm sure you'll find a doctor who will do it for you, unless you live remote.

One more thing you might want to consider are vaccine certificates.

Where I live certificates are valid for a year and booster shots renews a certificate. Also where I live one becomes eligible for a booster shot 6 months after final vaccine dose. So if one gets booster shot ASAP then one gets 18 months of valid certificate. If one delays booster shot until the last moment then one gets 24 month of a valid certificate.

And valid certificate is very useful over here so there is a real trade off between making one safer against infection vs making more actions available in the future.

I think it kind of sucks that this is a tradeoff one has to consider.

The Pfizer vaccine is not developed by Pfizer just like AstraZeneca's is not developed (as in decided on the formulation) by AstraZeneca. Both are big pharma companies. 

(both Moderna and BioNTech are what's traditionally called biotech companies and not Big Pharma)