On Friday, December 12, 2020, the United States Food and Drug Administration issued its first Emergency Use Authorization for a Covid-19 vaccine. The FDA’s approval of this vaccine, developed by Pfizer, followed recent approvals of the same vaccine by the UK, Canada, Bahrain, Saudi Arabia, and Mexico. Clinical trials have shown this mRNA-based vaccine, and a similar one produced by Moderna, to be 90-95% effective at preventing Covid-19 infections.

Over the next week, 2.9 million doses of the Pfizer vaccine will be shipped throughout the United States and first offered to health care workers, nursing home workers, and nursing home residents. The U.S. has suffered more than 298,000 deaths from Covid-19, with the worldwide total passing a staggering 1.6 million deaths. But now that an effective vaccine is on the way, many people find themselves worrying, “Is this safe?”

The name of the government program to create a Covid-19 vaccine (Operation Warp Speed), the politicization of the pandemic, and the speed at which these vaccines were created, have led many to question whether corners were cut to bring a vaccine to market so quickly. This article is one in a series that will address the vaccines, what we know about their safety profile, and the potential long-term effects of an mRNA vaccine.

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A December 2020 article in The New England Journal of Medicine detailed the safety and effectiveness of the Pfizer mRNA Covid-19 vaccine, based on a clinical trial of individuals ages 16 and older, who were either healthy or had chronic, but stable, medical conditions. These medical conditions included, but were not limited to, HIV and Hepatis B or C. Excluded from this trial were people with a medical history of Covid-19, a diagnosis of an immunocompromising condition, pregnant individuals, and those being treated with immunosuppressive therapy.

Pfizer’s mRNA vaccine comes in two doses, given 21 days apart. The clinical trial randomly assigned study participants to receive either the mRNA vaccine or a placebo. Neither the participants nor the staff administering the shots knew if they were getting the vaccine or the placebo.

43,548 participants from six countries, including the United States, were randomized to either the vaccine or placebo arms of this study. Participants included men (50.6%) and women (49.4%) and people who reported their race/ethnic group as White (82.9%), Hispanic/Latinx (28%), Black/African American (9.3%), Asian (4.3%), Multiracial (2.3%), Native American/Alaska Native (0.5%), Native Hawaiian/other Pacific Islander (0.2%), or not reported (0.6%). More than half (57.8%) were 16-55 years of age, while 42.2% were over the age of 55. Thirty-five percent of all participants were obese, with a body mass index of 30 or greater, and 21% had at least one coexisting health condition.

Most participants receiving the actual vaccine experienced short-term side effects that resolved within two days. These side effects occurred both in participants who received the vaccine and the placebo but were more frequent in the vaccine group. Mild to moderate pain at the injection site was the most commonly reported side effect, particularly in the younger subset of participants. Fatigue, headache, chills, muscle pain, and joint pain were also noted in both groups but were more common in the vaccine group. Fever, diarrhea, and vomiting were also occasionally observed but less common in both groups. These findings appear to be similar to those of other viral vaccines.

Pfizer was responsible for designing and conducting the trial and for collecting, analyzing, and publishing the data. Another company, BioNTech, worked with Pfizer and sponsored the trial and manufactured the vaccine. But, importantly, an independent data and safety monitoring board (DSMB) reviewed the efficacy and safety data. The DSMB is an independent group of experts whose job includes thoroughly reviewing the study data for quality, integrity, and participant safety, among other things. The members of this board must be independent from any professional or financial conflicts of interest with the research study or the study investigators. This step is critical in maintaining the integrity of the data review.

Related adverse events, or undesirable events, were rare and were reported in both the vaccine (21%) and placebo (5%) groups. These rare events included swollen or enlarged lymph nodes (0.3% vaccine, <0.1% placebo). Four related serious adverse events were reported in the vaccine group: shoulder injury related to vaccine administration, swollen lymph nodes in right armpit, irregular heartbeat, and pinched nerve in right leg.

This is not the only safety trial on mRNA Covid-19 vaccines. Other studies have also shown these vaccines to be safe overall and specifically in older adults.

The United Kingdom began administering the vaccine last week. Two National Health Service workers immediately experienced anaphylaxis, a serious allergic reaction that can include impaired breathing and a drop in blood pressure. Both individuals were treated and recovered, and both reported that they carry EpiPens due to histories of food allergies causing anaphylaxis. This adverse reaction prompted British officials to specify that the vaccine should not be given to anyone who previously had an anaphylactic reaction to food, medicine, or another vaccine. However, some experts think this may be an overreaction, and the expert advisory panels to the CDC will review the data and provide recommendations soon.

In addition to the data provided by the UK, Pfizer’s data reported to the FDA noted that 1 in 18,800 participants who received the vaccine had an anaphylactic reaction, compared to zero in the placebo group. Dr. Paul Offit, a well-known vaccine expert, noted that this rare reaction is treatable and easier to control than a severe case of Covid-19. He also stated that fewer than one in a million recipients of other vaccines have an anaphylactic reaction.

In short, this mRNA Covid-19 vaccine was found to be safe, and the Pfizer study included six months of follow-up data after the second dose. My next article in this series will examine what we know about long-term safety of mRNA vaccines.

If you receive this vaccine, expect that you may experience mild to moderate side effects, similar to those caused by many vaccines, for 1-2 days after your injection, so plan accordingly. This may mean planning your vaccine when you know you can rest and recover for 48 hours following your shot. If you have had an allergic reaction that caused a prior anaphylactic reaction, speak to your physician about whether you should get this vaccine and where.

Although people who were pregnant or breastfeeding were not included in this trial, the CDC notes that these individuals may still be candidates for this vaccine if they fall into a high-risk group. These individuals should discuss the risks and benefits with their physicians. The same advice holds true for people who are immunocompromised or taking immunosuppressive medications, particularly because these individuals have been shown to be at higher risk for a severe case of Covid-19 if infected.

All of us need to keep in mind both the pros and cons of this vaccine when deciding whether to get it. We are not weighing just the risks of the vaccine. We are weighing the risks of this vaccine against the risk of getting Covid-19. Although the survival rates from this virus are now higher than six months ago, particularly in people under the age of 70 and without other health problems, consider that many Covid-19 survivors experience long-term or permanent health problems (the “long-haulers”). Others may infect someone else who develops a more severe case. The hope is that people who take this vaccine will reduce not only their risk of getting Covid-19, but the risk of spreading this virus to others, thus putting a stop to this worldwide pandemic.

This article was written by Ellen Matloff for Forbes.com.