As the push to vaccinate as many Americans as possible against COVID-19 picks up steam, questions about the shots’ safety have often hindered the campaign.

Polling suggests that around 30% of the public does not plan to get vaccinated, the chief reason being concerns about the side effects, according to the Pew Research Center. Other surveys from Stanford University and the Kaiser Family Foundation have found evidence that fears of side effects are contributing to some hesitancy to get the shot.

Is there anything to worry about?

Since the drive began in December, more than 150 million COVID-19 vaccine shots have been administered. Behind the massive rollout is a safety monitoring effort that government scientists have described as “the most intense and comprehensive in U.S. history.”

The Times reviewed the scientific studies published thus far and interviewed half a dozen researchers. All the information points to the same conclusion: The shots are safe. And the small chance of experiencing a serious side effect from the vaccine is far outweighed by the danger of COVID-19.

Here’s a closer look at what the data show.

Serious reactions are rare

Scientists use a number of methods to look for potential warning signs that vaccines — whether they’re for influenza or COVID-19 — are causing serious side effects or death.

One of the most important tools in this effort is a government database known as the Vaccine Adverse Event Reporting System, or VAERS. The system allows patients — or their physicians, family members or lawyers — to submit a report of a health problem experienced after getting a shot.

VAERS is considered the nation’s early warning system, giving researchers the first indication that there may be a problem with a particular vaccine, said Dr. Tom Shimabukuro, deputy director of the Immunization Safety office at the Centers for Disease Control and Prevention and the vaccine safety team lead.

To date, the database contains just more than 50,000 reports of adverse reactions following the three vaccines currently authorized for emergency use in the U.S.: those from Pfizer-BioNTech, Moderna and Johnson & Johnson. Fewer than 8,000 of them are categorized as serious. That works out to a rate of about 50 reports per 1 million doses administered.

Six reported cases of blood clots in women following a Johnson & Johnson vaccination prompted the CDC this week to recommend a pause in the single dose vaccine. More than 6.8 million doses of the Johnson & Johnson vaccine have been administered in the U.S.

In other words, adverse reactions from vaccines are rare — especially when compared with the number of COVID-19 hospitalizations and deaths in the U.S.

At least 550,000 Americans have died of COVID-19, a rate of 1,700 deaths per 1 million residents. More than 2 million Americans have been hospitalized with COVID-19 since August. That’s over 6,200 patients per million.

But VAERS data are far from perfect.

Because the system relies on people to send in reports, the data it gathers are probably incomplete. Doctors and patients are less likely to document minor side effects than serious adverse reactions.

Also, VAERS is not designed to determine cause and effect. That means events can appear in the database without any medical evidence that firmly links them to the vaccine.

When interpreted properly, VAERS data can help reveal unusual or unexpected outcomes related to all vaccines administered in the U.S. For the COVID-19 vaccines in particular, scientists said they are monitoring VAERS and other data sources for any sign that the shots may be causing an uptick in a long list of health problems.

So far, no such signs have emerged, although researchers have detected very rare occurrences of anaphylaxis, a potentially life-threatening allergic reaction.

Each year, up to 100 people per 100,000 in the U.S. experience an anaphylactic reaction triggered by food allergies, insect stings, pollen, latex and other causes, and 1,000 people die.

Shimabukuro and colleagues used VAERS to identify a total of 66 cases of anaphylaxis following COVID-19 vaccines administered between Dec. 14 and Jan. 18. Each case was verified with medical records and, in some cases, follow-up interviews. With almost 18 million shots administered at the time, that comes out to a rate of 2.5 cases per million doses of Moderna and 4.7 cases per million of Pfizer. None of these cases were fatal.

These rates are comparable to those for other vaccines that are commonly administered to adults, including shots for influenza, meningitis and shingles. Past research suggests anaphylaxis can occur about once or twice per million doses across all vaccines.

“We have observed anaphylaxis following both of these vaccines, but anaphylaxis can occur after any vaccine,” Shimabukuro said. “It’s a known adverse reaction. In general, the safety profiles of these vaccines are consistent with what we expected based on the clinical trials, and they are reassuring.”

Some minor side effects are to be expected

Minor side effects such as headache, fatigue and fever appear to be more common after a dose of COVID-19 vaccine than, say, the annual flu shot, said Dr. Nicola Klein, director of the Kaiser Permanente Vaccine Study Center. They are not, however, surprising or unexpected.

“It means your body is appropriately responding to your vaccine and generating an immune response,” she said.

To help track these symptoms and more serious health outcomes, the CDC launched a program called V-safe that conducts “check-ins” with vaccine recipients via text message. Early data indicate the most common reactions include pain at the injection site, fatigue and headache — all similar to the flu shot.

Side effects for the second dose of the Pfizer and Moderna vaccines tend to be more powerful, experts said. V-safe respondents reported symptoms such as fatigue, muscle pain, headache and fever at higher rates after their second shot of Pfizer than the first. Data for the second dose of Moderna were not available.

Dr. Grace Lee, a pediatrics professor at Stanford University, said most of the side effects that recipients reported in V-safe were the kinds of short-lived reactions that are typical after vaccinations.

Lee, who is a member of the U.S. Advisory Committee on Immunization Practices and California’s COVID-19 Scientific Safety Review Workgroup, compared the reactions to the kinds of things warned about in “What to Expect When You’re Expecting,” a popular book about pregnancy. Some minor ill effects are natural and ultimately harmless, like feeling nauseated during pregnancy.

“This is a normal way people should feel after getting a vaccine, and there’s nothing wrong with it,” Lee said. “We don’t want people to feel like something happened to them, or that they’re having a bad reaction to the vaccine.”

Signals in the noise

Though VAERS and the V-safe app for the COVID-19 vaccines provide valuable data, the numbers alone are impossible to interpret without additional context.

Consider a fraction. VAERS supplies only the number on top, the numerator. Here, that’s the number of people who got vaccinated, experienced an adverse reaction and then reported it to VAERS.

“This isn’t enough information to determine if a vaccine caused an adverse event, but it allows us to detect potentially concerning patterns,” Shimabukuro said. “We have other more sophisticated systems to evaluate cause and effect.”

What VAERS doesn’t provide is the number on the bottom of the fraction, the denominator. In most cases, that should be the number of people who have been vaccinated, Shimabukuro said. To draw any meaningful conclusions, researchers must find this number elsewhere.

As part of its early monitoring effort, the CDC partnered with the Brown University School of Public Health to monitor the safety of the COVID-19 vaccines as they were distributed to residents of long-term care facilities across the country.

Between Dec. 21 and Jan. 18, VAERS received 129 reports of nursing home deaths following vaccination for COVID-19. To determine whether these deaths were caused by the vaccines, researchers had to find their denominator.

One approach is to compare the reported number of a particular type of event — be it deaths or a certain side effect — with the number of times the event would occur in the absence of vaccination. Scientists call this the background rate.

To determine the background rate of deaths in care facilities, the researchers first estimated how many residents received vaccines during the 29-day period. Based on CDC records, about 2 million doses were administered in such facilities. They assumed 1.3 million went to residents, with the rest going to staffers.

Prior research suggests the mortality rate at long-term care facilities is as high as 22% in a typical year. That means over 29 days, about 11,440 deaths could be expected due to natural causes.

2 million vaccination doses in

long-term care living facilities

1.3 million residents vaccinated

over 29-day risk period

11,440 deaths expected

2 million vaccination doses in

long-term care living facilities

1.3 million residents vaccinated

over 29-day risk period

11,440 deaths expected

2 million vaccination doses in

long-term care living facilities

1.3 million residents vaccinated

over 29-day risk period

11,440 deaths expected

The 129 reported deaths in VAERS are well below the expected background rate, Shimabukuro said. That means it’s “highly unlikely” the vaccine is causing or contributing to any additional deaths, he said.

11,440

expected deaths unrelated to vaccines

129

VAERS reports of deaths

11,440

expected deaths unrelated to vaccines

129

VAERS reports of deaths

11,440

expected deaths unrelated to vaccines

129

VAERS reports of deaths

To determine the background rate of deaths in care facilities, the researchers first estimated how many residents received vaccines during the 29-day period. Based on CDC records, about 2 million doses were administered in such facilities. They assumed 1.3 million went to residents, with the rest going to staffers.

Prior research suggests the mortality rate at long-term care facilities is as high as 22% in a typical year. That means over 29 days, about 11,440 deaths could be expected due to natural causes.

The 129 reported deaths in VAERS are well below the expected background rate, Shimabukuro said. That means it’s “highly unlikely” the vaccine is causing or contributing to any additional deaths, he said.

2 million vaccination doses in

long-term care living facilities

1.3 million residents vaccinated

over 29-day risk period

2 million vaccination doses in

long-term care living facilities

1.3 million residents vaccinated

over 29-day risk period

11,440 deaths expected

11,440

expected deaths unrelated to vaccines

129

VAERS reports of deaths

A vast network

The United States relies on a massive system of interconnected databases and multiple agencies to monitor vaccine safety. Engaged in the COVID-19 effort are the CDC and Food and Drug Administration, as well as the U.S. Department of Veterans Affairs, the Department of Defense and the Centers for Medicare and Medicaid Services.

One network, called the Vaccine Safety Datalink, or VSD, is a collaboration between the CDC’s Immunization Safety Office and nine healthcare organizations. It uses the electronic health records of about 12 million patients to conduct near real-time surveillance.

“We look at the data every week and look to see whether there’s any sort of concerns or anything that we need to worry about,” said Klein, who leads the VSD’s COVID-19 vaccine safety surveillance study.

The VSD system gives researchers a more robust look at a patient’s health before and after vaccination — something VAERS and the newly implemented V-safe app cannot do. In 2008, VSD allowed Klein’s team to detect an increased risk of febrile seizures in toddlers who received a combination vaccine for measles, mumps and rubella and chickenpox. The discovery led to policy changes from the CDC.

Safety monitoring will continue and expand as more vaccines are administered and authorized for use. Separate studies through the VSD and the V-safe app are also monitoring outcomes of women who are vaccinated while pregnant and of their infants after they are born.

The data on the vaccines in general are reassuring.

“With all of these systems in place, we’re really just not seeing anything of concern right now,” said Allison Naleway, an investigator with the VSD and Kaiser Permanente’s Center for Health Research. “So that’s good news for our public.”