Last month, the Food and Drug Administration approved the Pfizer-BioNTech Covid 19 vaccine, making it the first Covid-19 vaccine to move beyond emergency use authorization.
The fact that the vaccine now has full FDA approval doesn’t seem to matter to many of the people I talk to. They remain concerned about potential long-term effects from the Covid-19 vaccine that clinical trials might have missed or not run long enough to catch, and bring up past incidents in which the FDA approved drugs that were later recalled for dangerous side effects. Here are some things to know about vaccine side effects:
1. Vaccines rarely, if ever, cause long term side effects because they aren’t in the body long enough
Although the Covid-19 vaccines went through all the same safety trials as any other vaccine (just much faster, courtesy of Operation Warp Speed), they haven’t been around for long. When the FDA issued its emergency use authorization for the Moderna vaccine in December of 2020, the longest any clinical trial participant had been vaccinated was about nine months. For Pfizer, that number was closer to eight months. It only became clear that the vaccines could be linked to side effects like Guillain-Barré Syndrome and blood clots after clinical trials were over. So how do we know unexpected, damaging side effects won’t still emerge?
The reason is that vaccines are very different from other common medicines, many of which we take on a regular basis for months or even years. Due to how often we take them, these medicines can build up in the body over time, causing side effects far down the road. One of many common examples is how NSAIDs like aspirin and ibuprofen can cause kidney and liver damage over time.
In contrast, the ingredients in a vaccine only stay in the body for a few days at most. This is because all vaccines, including the Pfizer and Moderna Covid-19 vaccines, are purposefully made out of materials your body will recognize and know to degrade. For instance, mRNA is used constantly by every cell in your body as a temporary memo, delivering instructions encoded in your DNA to your cells as conditions change (more on that here). Once its message is read out by your cells, the mRNA is degraded so it won’t be confused with new, updated instructions. On average, mRNA can last for only a few hours inside the cell. In addition, the SARS-CoV-2 spike protein encoded by that mRNA, and any other parts of the vaccine, are quickly recognized by your body as foreign, and cleared out by the immune system. This is the whole point of vaccination: it provides a safe practice target for your immune system to seek out and destroy. The process of destroying the vaccine trains your body to look out for the virus if you are infected again, thus protecting you from getting sick. (More about the immune system here and here.)
The fact that vaccines are so quickly cleared from our systems explains why, in the entire modern history of vaccination, we almost never see serious side effects occurring more than a few weeks after vaccination. This is why the FDA is comfortable approving vaccines after two months of clinical trials. Longer term side effects could theoretically be possible, even though we’ve never seen them. However, when it comes to the Covid-19 vaccines, over 200 million people worldwide were vaccinated over six months ago, with no long-term side effects found. We’re well out of the woods.
2. Short term side effects are also extremely rare, and often predictable.
While no long-term side effects of the Covid-19 vaccine have been found, we have of course seen short term side effects. Aside from the typical affects you would expect upon exercising your immune system via vaccination (fever, chills, headache, soreness, etc), we’ve seen rare instances of blood clots, Guillain-Barré syndrome, and anaphylaxis within a few minutes to two weeks after vaccination. These side effects are extremely rare. They are also something scientists were looking out for: blood clots, Guillain-Barré syndrome, and anaphylaxis have all been seen at equally minute levels for some other vaccines in the past. The only reason we didn’t see them in initial clinical trials prior to FDA approval is that they are so rare, they didn’t even occur in the thousands of trial participants. For example, the 0.00009% chance of blood clots from the J and J vaccine is so low, that out of the 43,000 stage three participants, only 0.04 “people” would be expected to experience the clots.
3. The FDA has an impressive record at catching vaccine side effects before approval
A number of people I’ve talked to don’t want anything to do with a Covid-19 vaccine regardless of FDA approval or emergency use authorization. They point to FDA-approved drugs that were later recalled due to damaging side effects, or cases in which vaccine batches were recalled over manufacturing concerns.
These are fair arguments, but when we look at the numbers, the FDA has an impressive record at catching dangerous vaccines before approval. In fact, FDA clinical trials are so good at picking up on potentially dangerous side effects, that since the 1950s, only one vaccine was pulled off the shelves after approval. And that side effect was barely above baseline levels.
The vaccine, Rotashield, was approved in 1998 to combat rotavirus infection. In very rare cases, it can cause intussusception in infants (a type of bowel obstruction). The issue always occurred within two weeks of the shot, and was likely missed in clinical trials due to how rare it was: almost the same number of infants in the placebo group had the same complication, which does occur occasionally in infants for reasons other than vaccination. Separate analysis concluded that Rotashield did increase the chance of intussusception in infants (by 2 cases per 10,000 infants), and because the most severe complications of rotavirus can be managed in the United States, the FDA pulled the vaccine in our country.
Although FDA-approved medications we take on a regular basis have been recalled, the FDA has an impressive record of catching side effects in vaccines before approving them. You can see a list of every vaccine-related incident since the 1950s here: aside from Rotashield and another vaccine linked to Guillain-Barré syndrome, all were recalls of specific vaccine batches over manufacturing concerns.
4. Vaccine side effects are very closely monitored
An obvious hole in the argument I’ve made above is that side effects (long or short term) might just seem rare because we haven’t looked hard enough for them. The Vaccine Adverse Event Reporting System (VAERS) keeps a comprehensive list of potential vaccine side effects observed by manufacturers, medical professionals, scientists, patients, and family members. Although many reactions seen after vaccination are not actually caused by the vaccine itself, these reported reactions are monitored weekly to catch real potential side effects. We know that VAERS is monitored unbelievably closely because last April, health professionals and scientists were able to identify blood clots as a potential short term side effect of the Johnson and Johnson Covid-19 vaccine from just 6 VAERS entries. This close monitoring is also how the potential link of Rotashield to intussusception was found in the late 1990s.
When a potential side effect is found, it’s tested, as we saw for the Rotashield vaccine. If we can actually say that vaccination increases the risk of that effect, and it is bad enough to outweigh the benefits provided by vaccines, it gets pulled from shelves.
Side effects like Guillain-Barré syndrome or anaphylactic shock are undoubtedly scary. As humans, we tend to latch onto scary events, no matter how rare they are. But as with anything else in life, no option is risk free, and when it comes to vaccines, the risks are indeed low.
Hannah Margolis is a NSF GRFP Fellow at the Massachusetts Institute of Technology with a degree in biochemistry from Dartmouth College. She can be reached at hannah.k.margolis @gmail.com.