Vaccines are highly unlikely to cause side effects long after getting the shot


Seven months after the U.S. began administering COVID-19 vaccines, the latest figures from the nonprofit Kaiser Family Foundation’s ongoing tracking poll show that 10 percent of adults are still nervous about the vaccine and want to “wait and see” how others fare before rolling up their sleeves. Young adults ages 18 to 29 and Black and Hispanic people are some of the most likely to voice this sentiment.

The main two reasons cited for this hesitancy are that the vaccines are “too new” and that they may trigger unexpected or life-threatening side effects, perhaps even months or years later. It’s true that reports of new side effects can sometimes take months to emerge as a vaccine goes from populations of thousands in clinical trials to millions in the real world, encountering natural variations in human responses along the way. But more than a hundred million Americans have already passed that point in their vaccinations and the first participants in the clinical trials are now beyond a year.

So far, incidents of severe side effects for the coronavirus vaccines such as Guillain-Barré Syndrome and heart inflammation are very rare, and they were discovered quickly because they were on official lists of potential problems to watch for. What’s more, all these and other side effects appear soon after someone has taken the vaccine, suggesting that people don’t need to worry about delayed long-term reactions.

This picture fits with the modern history of vaccinations, which shows that most new immunizations have been incredibly safe, and even the most severe effects have reared their ugly heads right away.

“Side-effects nearly always occur within a couple of weeks of a person being vaccinated,” says John Grabenstein, director of scientific communication for the Immunization Action Coalition. He adds that the longest time before a side effect appeared for any type of shot has been six weeks.

“The concerns that something will spring up later with the COVID-19 vaccines are not impossible, but based on what we know, they aren’t likely,” adds Miles Braun, adjunct professor of medicine at the Georgetown University School of Medicine and the former director of the division of epidemiology at the U.S. Food and Drug Administration.

A key reason for this limited window of side effects is the short time all vaccines stay in the body, says Onyema Ogbuagu, an infectious diseases specialist at Yale Medicine and a principal investigator of the Pfizer-BioNTech COVID-19 vaccine trial. Unlike medicines that people take every day or week, vaccines are generally administered once or a handful of times over a lifetime. The mRNA molecules used in the Pfizer and Moderna vaccines are especially fragile, he notes, so “they are out of your body in a day or so.”

The vaccines subsequently get to work stimulating the immune system so it can memorize the virus’s blueprint and mount a quick response if it encounters the real thing later. “This process is completed within about six weeks,” says Inci Yildirim, a vaccinologist and pediatric infectious diseases specialist at Yale Medicine. That’s why serious adverse effects that might be triggered by the process emerge within this time frame, after which everything is put on a shelf in the body’s library of known pathogens, Yildirim says.

A stroll through vaccine history confirms that even the most damaging side effects have indeed taken place within a six-week window.

After the initial Salk polio vaccine was introduced in 1955, it became clear that some of the first batches inadvertently contained live polio viruses and not the weakened form intended to be in the shot. Within weeks, this mistake resulted in some polio infections and, in a few cases, eventual death. The “Cutter incident,” named after the manufacturing labs with the biggest mishaps, prompted more stringent government regulations. Today, polio vaccines are monitored to make sure the virus is completely inactivated in shots given to children.

In 1976, rare cases of the nerve disorder Guillain- Barré Syndrome emerged some two to three weeks after people began receiving an egg-based inactivated flu vaccine against a dangerous strain of H1N1 swine flu. Scientists eventually determined the effect occurred in one to two people per million shots. Guillain- Barré is a treatable disease, but with flu season winding down that year, the vaccination effort was soon abandoned.

So, no vaccine has caused chronic conditions to emerge years or decades later, says Robert Jacobson, medical director of the population health science program at the Mayo Clinic. “Study after study have looked for this with all sorts of vaccines, and have not found it to be the case,” he says.