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The United States is hurtling toward an ominous winter, as coronavirus cases, hospitalizations, and deaths set bleak new records with few signs of abating. But a glimmer of hope is on the horizon: The Food and Drug Administration is poised to approve a pair of coronavirus vaccines that have efficacy rates of roughly 95 percent and have shown no serious side effects in clinical trials.

By the end of this month, millions of people across the country could be inoculated against COVID-19. Yet skepticism about the vaccines persist. Recent polling by the Pew Research Center suggests 60 percent of Americans would take a coronavirus vaccine if one were available today, but the rest remain wary, questioning the vaccines’ side effects and long-term safety.


In clinical trials of tens of thousands for these vaccines — one developed by Pfizer in partnership with the German company BioNTech, and the other, by Cambridge-based Moderna — participants reported mild to moderate side effects, such as soreness, muscle aches, fever, and fatigue that typically resolved within one to two days.

An independent analysis of Moderna’s trial by the Data and Safety Monitoring Board, an independent group of experts appointed by the National Institutes of Health, found side effects were more pronounced after the second dose, and included fatigue in 9.7 percent of participants, muscle aches in 8.9 percent, joint pain in 5.2 percent, and headache in 4.5 percent. In Pfizer’s trial, following the second shot, 2 percent of recipients experienced headache and 3.8 percent felt fatigue. Older adults in Pfizer’s study also reported fewer and less severe reactions than younger people, owing, perhaps, to their weaker immune response.

“Because it’s not been seen in the trials, we’re not expecting to see people end up with any kind of severe side effects,” said Dr. Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women’s Hospital. “We’re not expecting that people are going to have problems with their liver, their kidneys, their heart or anything like that.”


The United Kingdom’s medical regulatory agency is advising people with a “significant history of allergic reactions” not to receive the vaccine after two National Health Service staffers showed symptoms of “anaphylactoid reaction” after getting their first shot on Tuesday.

Long-term side effects of the vaccines are unknown, but based on the evidence, virologists and infectious disease specialists believe serious complications from the leading vaccine candidates are unlikely. However, continued surveillance of those who’ve been immunized will be crucial.

The drugmakers will continue monitoring trial participants for two years after the second dose. Members of the public also can report side effects to the Vaccine Adverse Events Reporting System, a national surveillance program run by the FDA and Centers for Disease Control and Prevention that detects possible vaccine safety issues.

“Biologically, we have very little reason to believe there are any long-term safety issues,” said Natalie Dean, a biostatistician specializing in infectious diseases and vaccine study design at the University of Florida. “Usually, if a vaccine has some sort of safety issue, it would pop up shortly after the time of vaccination. It would be like some over-reactive autoimmune response that happened.”

Pfizer and Moderna’s coronavirus vaccines both use new messenger RNA, or mRNA, technology to trigger an immune response. A molecule found in the nucleus of human cells, mRNA is responsible for making the proteins our bodies need to survive. The Pfizer and Moderna vaccines work by injecting synthetic mRNA into the cells, giving them instructions for creating a piece of protein found on the surface of the novel coronavirus. The immune system recognizes this protein as an intruder and begins producing antibodies that protect the body from infection.


There are currently no licensed mRNA vaccines in the United States, but scientists have been studying the possibility of using mRNA to fight disease for decades.

“This isn’t some freaky thing that doesn’t exist ...This is copying the beauty of biology,” Paul Duprex, director of the Center for Vaccine Research at the University of Pittsburgh, said of the mRNA vaccines.

“They’re not going to integrate into your genetic material and become you. They’re not microchips planted by Bill Gates that are going to track you as you drive down the 93,” he added. “But they are things that have been created by people to fight the disease, just the same way as antibiotics were created and formulated by people to fight disease.”

While there’s still much to be learned about the effectiveness and durability of these vaccines, Duprex, along with other public health experts, believe the benefits of getting vaccinated outweigh the potential risks.

“Because vaccines are so well tested, it’s technically much more dangerous, much more risky to get the disease than it is to get the vaccine,” he said. “And I would far rather have headache, muscle soreness, fatigue, chills, nausea, and a bit of pain at the injection site than be in intensive care with a ventilator.”


Deanna Pan can be reached at deanna.pan@globe.com. Follow her on Twitter @DDpan.