Everyone receiving the COVID vaccines is asked to wait for 15 minutes in case they have a bad reaction to the shot. Given that we've now had hundreds of millions of Pfizer vaccine shots injected around the world, is there any data showing how many of those people end up needing medical attention during those 15 minutes (possibly 20 minutes in some areas, as reported in comments)?

If such data is available, do we know how many of those people ended up needing attention because of the vaccine itself? I.e. if you take any random 15 minute period, around 23 Americans will get a heart attack during that time, so logically speaking not all cases of medical problems during the 15 minute period will be caused by the vaccine itself.

Note that I'm only interested in that exact 15-minute waiting period, not any other side effects from the vaccine that are detected later on. I.e. myocarditis is suspected to be linked to the COVID shot for some people, but that's usually detected later on, not during the 15 minute observation period.

  • Just yesterday night I thought to post this in MedicalS.SE! Commented Nov 9, 2021 at 6:38
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    @chrisH @johmathanReez that’s not standard for the U.K. it’s 15 minutes. Not sure why Chris had to wait 20. nhs.uk/conditions/coronavirus-covid-19/coronavirus-vaccination/… `
    – Tim
    Commented Nov 9, 2021 at 19:50
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    @Tim I have a feeling it was 20 for the first dose (May), 15 for the 2nd (July), so maybe the guidance changed. I know the myocarditis worry came along about the time of my 2nd dose, because I had a massive bike ride starting 49 hours after it.
    – Chris H
    Commented Nov 9, 2021 at 20:52
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    This is anecdotal, so not worth a whole answer, but when I helped a friend of mine get her shots, I asked the attending nurse specifically this question while waiting. In this case, they were doing them in a tent, outside, in the midday Florida summer heat. Nearly all of the people who needed medical attention were simply cases of heat exhaustion from being forced to wait outside for that long. My own shots were thankfully done indoors, in an air-conditioned pharmacy. Commented Nov 10, 2021 at 18:20
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    @JonathanReez If you want to make alarming claims, you need to bring convincing evidence. Comments here aren't a discussion forum.
    – Carey Gregory
    Commented Nov 11, 2021 at 6:45

3 Answers 3


The most noteworthy complication, and most heard about in the news, is anaphylaxis.

This article breaks down cases of anaphylaxis nicely: Reports of Anaphylaxis After Receipt of mRNA COVID-19 Vaccines in the US—December 14, 2020-January 18, 2021. They provide a great table with a specification of whether the case happened during the 15-minute window:

enter image description here

So for Pfizer specifically the numbers are:

  • 4.7 anaphylaxis cases per million in total
  • 3.6 cases per million if only the 15 minute window is considered
  • 1.1 per million for those without a prior allergic reaction to a drug or vaccine

This doesn't account for non-anaphylaxis allergic reactions (more common, less severe), vasovagal syncope(fear of needles/injections), or any other, more rare conditions.

To date data regarding medical attention received within 15 minutes of vaccination has been hard to come by. The data from the first week of Pfizer administration, covering some 1.8m patients shows that non-anaphylaxis allergic reactions were roughly 8 times more common (and typically far less severe) than anaphylactic reactions.

While we could attempt to extrapolate based on anaphylaxis cases and non over the year based on the first week's data, it'd be a relatively pointless exercise. The occurrences/million for anaphylaxis dropped from 11.1 to 4.7 in the time frame. We can't assume the reduction in case numbers to be proportionally identical, yet a simple extrapolation based only on time would be disingenuous.

In short, reliable data on the topic outside anaphylaxis is hard to come by because other instances requiring medical attention are either less severe or less common.

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    I wonder where the massive gender imbalance comes from - it's unlikely to be reporting bias for something serious happening under observation
    – Chris H
    Commented Nov 9, 2021 at 15:47
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    @TCooper but those effects would be present even if you injected a saline solution into each patient. Other effects (like random heart attacks) would be present even if you didn't inject anything at all and just had people sit in a room for 15 minutes. If you know of a good paper that takes this into account, I'm happy to see the answer! Commented Nov 9, 2021 at 23:31
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    @Joshua: What biochemical 'knowledge' did you search? A quick google search brings up this paper that says "One possible explanation for the sex imbalance is that sensitization to PEG is more common in women due to the relatively frequent exposure to PEG-containing products, such as cutaneous exposure to cosmetics or the use of medications such as contraceptive injections.". Whether it is correct or not is not my point; you shouldn't anyhow claim that the data is not trustworthy without evidence against it.
    – user21820
    Commented Nov 10, 2021 at 7:43
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    @ChrisH I suspect a significant part of the observed gender imbalance is because this is based on data only up to mid-January 2021. That and the age range suggests that these were mostly vaccinations of nurses and other (health)care personnel, where women tend to be overrepresented in general. To draw any conclusions wrt gender imbalance, we would have to examine risk ratios (events normalized to number of doses administered per gender).
    – TooTea
    Commented Nov 10, 2021 at 10:47
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    @ChrisH: I can't account for it either, but the initial report for blood clots in J&J showed 100% females affected, which also turned out to be bad data, as in not suitable for further induction.
    – Joshua
    Commented Nov 10, 2021 at 15:12

The data on this aren't hard to find. Here are some more examples that specifically provide time frames to back up @A Rogue Ant's answer:

For the Pfizer vaccine:

During December 14–23, 2020, monitoring by the Vaccine Adverse Event Reporting System detected 21 cases of anaphylaxis after administration of a reported 1,893,360 first doses of the Pfizer-BioNTech COVID-19 vaccine (11.1 cases per million doses); 71% of these occurred within 15 minutes of vaccination.

And for Moderna:

During December 21, 2020–January 10, 2021, monitoring by the Vaccine Adverse Event Reporting System detected 10 cases of anaphylaxis after administration of a reported 4,041,396 first doses of Moderna COVID-19 vaccine (2.5 cases per million doses administered). In nine cases, onset occurred within 15 minutes of vaccination. No anaphylaxis-related deaths were reported.

In both studies, 70-90% of anaphylactic reactions occurred within the first 15 minutes, hence the 15-minute protocol.

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    Why is all the data specific to anaphylaxis? What about other side effects?
    – Barmar
    Commented Nov 9, 2021 at 15:44
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    @Barmar Like what? They report the side effects that actually occur.
    – Carey Gregory
    Commented Nov 9, 2021 at 15:51
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    I guess I find it hard to believe that there have been no other types of adverse reactions?
    – Barmar
    Commented Nov 9, 2021 at 15:53
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    @Barmar What other types of adverse reactions would you expect? Commented Nov 9, 2021 at 15:54
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    @Joshua Calling data junk based on your anecdotal experience is also junk.
    – Carey Gregory
    Commented Nov 9, 2021 at 15:58

For the UK up to 27 October 2021:

All recipients were requested to wait 15 minutes before departing the point of vaccination.

A history of anaphylactic reactions to any of the ingredients forbids taking the vaccine, but which dose (first or second) the listed reactions occurred after is not specified in the data.

Estimated first/second doses - Vaccine Manufacturer - Anaphylaxis or anaphylactoid reactions.

23.5/20.3 million doses - Pfizer/BioNTech vaccine - 517

24.9/24.1 million doses - AstraZeneca - 834

1.5/1.3 million doses - Moderna - 41

From UK Government website 8th November 2021.

With reference to the total number of reactions reported:

Pfizer/BioNTech - 357,084 (Anaphylactic data on page 31)

Astra Zenica - 836,957 (Anaphyl. data page 37)

Moderna - 55,081 (Anaphyl. data page 18)

A small number of reports from unspecified vaccines with 1 anaphylactoid reaction for 3,557 reports.

  • Sounds like this includes all allergic reactions, not just reactions detected during the 15 minute wait? Commented Nov 8, 2021 at 23:42
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    I'm having second thoughts about the way data is differentiated in the UK. It's not clear at all from the figures that the 15 minute window is what's listed in that field (especially in light of the per-million figures for the US given by the other answers). @JonathanReez Commented Nov 9, 2021 at 2:15

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