Should I be worried about potential long-term side effects of COVID-19 vaccines?


A: Realistically, no. Biologically, the chances of long-term effects are vanishingly small.

TL;DR: Serious vaccine side effects are exceedingly rare and occur hours or days after vaccination, not months or years.

The risk of infection and long-term health effects of COVID-19 infection are high and very real.

We *really* feel the sentiment behind this common vaccine question. After all, the vaccines were developed at “Warp Speed” and we have only followed people up for a short time. How can we be sure there are no long-term side effects of the vaccine?

While the Nerdy Girls continue to emphasize that science is a *process* rather than a specific answer, the scientific process has delivered much data that can ease our minds about unknown long-term effects.

📜 We’ve learned a lot from a long history of vaccine science.

Most vaccine side effects occur *very soon* after vaccination, within minutes to hours (allergic reactions), or within days/weeks (fever, body aches). Severe adverse events are very rare. The current trials have enrolled tens of thousands of participants since March 2020 so we have had a lot of time to see reactions. No serious adverse events were observed in the Pfizer and Moderna trials. While a very small number of treatable allergic reactions have been seen out of the millions of vaccines deployed so far, the number of reported adverse events has been very small and lower than normal expected rates in the population.

🙋 But haven’t there been scary long term effects of vaccines before?

Historically, the occurrence of “latent” or long-term effects of vaccines that show up much later has only been seen *very rarely* for vaccines that use LIVE viruses including the oral polio vaccine (OPV) and live varicella (chickenpox) vaccines. Varicella is a type of herpesvirus, which are “special” in that they can undergo latency and later reactivate in the body—think shingles and cold sores. Notably this type of reactivation is much more common from varicella *infection* than vaccination.

The current Pfizer and Moderna mRNA vaccines contain NO VIRUS whatsoever. They contain instructions for making a single protein from SARS-CoV-2 that elicits an immune response to recognize and respond quickly if it encounters the virus in real life. There is thus NO chance that the virus could remain latent and reactivate like shingles….because the vaccine contains no virus.
The mRNA disappears very rapidly from the body (within hours), like a vanishing snapchat message. This means there is no mRNA hanging around your body to cause unexpected problems later—very different from a drug that you may take regularly for months or years.

One possible long-term effect is a phenomenon called “antibody dependent enhancement” or ADE. This occurs when antibodies for an infection or vaccine actually make subsequent infection with another type worse. Dengue virus re-infection is the primary example of this phenomenon. Scientists have been on the lookout for ADE in SARS-CoV-2, but there has been no evidence of ADE observed for SARS-CoV-2 either from the vaccine trials or from natural re-infection. Read our recent post on ADE here.

Guillain-Barré syndrome (GBS) is a rare disorder where the body’s immune system damages nerve cells and can cause temporary muscle weakness or paralysis. The syndrome typically follows viral or bacterial *infection*, but in rare cases can follow vaccination (after days or weeks, not months). Most people fully recover from GBS. There have been NO cases of GBS reported from any of the COVID-19 vaccine trials.


➡️ Long-term effects of vaccines are exceedingly rare, and not biologically plausible for current COVID-19 vaccines.
➡️ Most side effects or adverse events happen within HOURS or DAYS.
➡️ Vaccines don’t stick around in your body like medications you take repeatedly.
➡️ The risk from natural infection is much higher than the risk of extremely rare side effects.
➡️ Besides the risk of short-term illness, hospitalization or death, remember the potential long-term effects of COVID infection which include: chronic fatigue, brain fog, blood clots, cardiac, lung and kidney damage.
➡️ The most common long-term effect of vaccines is immunity and protection against disease.

Stay safe and stay sane!

Those Nerdy Girls

Good review of why the COVID-19 vaccine process has not been as rushed as it seems

UK vaccine “Yellow Card” reporting

“COVID-19 vaccine safety update US” (presentation slides from Jan 27th ACIP meeting)

Coverage of “Long Covid”

More on ADE

Guillain-Barre syndrome

“What are the Long Term Safety Risks of the Pfizer and Moderna Covid-19 Vaccines?”

“COVID-19 vaccines are safe, even with long-term data lacking”

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