This is in regards to the COVID-19 vaccines.

As far as I am concerned, the speedy availability is the conjunction of 3 things coming together:

  • superior biotech compared to even 10 years ago, both in genomic analysis and vaccine tech;
  • running all the paperwork in tandem with the development, rather than sequentially;
  • lots of money and resources and just the highest priority.

Some people are concerned, and say that this is so new that we don't know about long-term side effects.

What are the historical examples where significant harmful side effects showed up long after a Phase 3 trial of a vaccine?

The examples I've been able to uncover are:

  • A vaccine issue in the Philippines in 2017: dengvaxia. Dengue vaccines can under certain conditions make things worse if you catch a different strain, but I am not sure it came to light after initial trials gave the OK or if the trials were run well enough to catch this effect. I assume that with the large-scale attention given to the various covid vaccines, we would have caught them out if they reinforced infections instead.

    Dengvaxia is approved for sale in 19 countries, including the European Union and the United States, and has saved an estimated 1 million lives. ...

    No one reportedly died from the vaccine. Many of the victims' parents nonetheless blamed the dengue vaccine for their children's deaths.
    Dengvaxia controversy - Wikipedia

  • There are plenty of cases where things go wrong in medical trials or with released medication, for example, thalidomide. Or medications that caused problems which only became apparent over time. In that list, only one is a vaccine which caused problems with infants who are NOT covid vaccination targets.

Unlike a regular medication, it would seem to me the problems with a vaccine should become apparent relatively quickly after administration, as you don't keep on taking it. Additionally, the medical community has been pretty clear that they don't advocate giving this vaccine to young children and are not committing much to pregnant women either.

So, historically, aside from the dengvaxia - which is not a clear-cut failure either as it seems to protect more than it hurts - do people who claim dire risks have anything concrete to back their statements?

Vaccines have been the subject of disinformation campaigns in the past, but on the other hand, there have been cases with non-vaccine medications where, in hindsight, the signs of adverse long term effects weren't sufficiently taken into account.

  • 9
    The reason we got these vaccines so quickly is that they've been working on a Coronavirus platform vaccine since SARS and MERS (also coronaviri), so they were already pretty far along. I'd call it luck, but I'd imagine the families of those hit by those two diseases don't feel so lucky. Its more like career virologists doing what they do for decades with little funding or fanfare until suddenly it became the most important effort on earth.
    – T.E.D.
    Dec 16, 2020 at 13:51
  • Comments are not for extended discussion; this conversation has been moved to chat.
    – T.E.D.
    Dec 16, 2020 at 15:33
  • @gktscrk Yes, since a Phase 3 trial was probably a result of bad past experiences. Maybe adding a bit of research on how that came about would heighten the quality of the question (in a historical context). Dec 17, 2020 at 5:55
  • @T.E.D. I agree, count that as my point #1 of why it was fast. But my question is really not about why it was quick. Merely about examples of actual vaccine problems discovered long after modern well-conducted phase 3 trials. Dec 17, 2020 at 18:52

1 Answer 1


It pains me to give the antivaxxers the slightest ammunition, but an example of a vaccine that can have long-term, negative effects some time after it has been administered is the Oral Polio Virus.

A potential, adverse effect of the OPV is its known ability to recombine to a form that causes neurological infection and paralysis. This genetic reversal of the pathogen to a virulent form takes a considerable time (at least 12 months) and does not affect the person who was originally vaccinated. The vaccine-derived attenuated virus is normally excreted from vaccinated people for a limited period. Thus, in areas with poor sanitation and low vaccination coverage, the spontaneous reversal of the vaccine-derived virus to a virulent form and its spreading in the environment can lead to unvaccinated people becoming infected.

John Kolmer's 1935 polio vaccine was given to ten thousand children, five of these died of polio and 10 more were paralysed, usually in the arm where the vaccine was injected

Even Jonas Salk's polio vaccine could cause problems, though this was because of poor preparation. The Cutter Incident was due to live virus being allowed to contaminate the vaccine.

In spite of these problems, the vaccines have hugely reduced the deaths and disabilities caused by polio. In 1952, a polio epidemic in the USA caused 58,000 cases including 3,145 deaths and 21,269 cases of paralysis. The USA has been free of polio since the 1960s

  • Nice example. But this would not apply to the mRNA versions of the covid vaccines as those don't use a functional virus. As to your concern: lack of info is the biggest problem - if the worst actual case we have to date is this one example, which doesn't even apply to a number of the Covid vaccines, then that risk seems light against 1.78M deaths and counting. Dec 29, 2020 at 20:15

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