Two years into the COVID-19 pandemic, the U.S. healthcare system is on a historic drive to vaccinate as many people as possible — an effort that has gained urgency with the arrival of the highly infectious new omicron coronavirus variant.

Everyone in the United States age 5 and above is currently eligible for a COVID-19 vaccine. Children age 12 and up and adults who are fully vaccinated are also eligible for a booster shot.

“Although we don’t have all the answers on the omicron variant, initial data suggests that COVID-19 boosters help broaden and strengthen the protection against omicron and other variants,” said Rochelle Walensky, MD, MPH, director of the Centers for Disease Control and Prevention (CDC), in a statement in December 2021.

Three vaccines are available in the United States: the Pfizer-BioNTech and Moderna vaccines, which require two shots timed about a month apart; and the Johnson & Johnson (J&J) vaccine, a one-shot immunization from the company’s Janssen subsidiary.

The Pfizer and Moderna vaccines have received full approval from the Food and Drug Administration, while the J&J vaccine has been granted emergency use authorization.

Here’s what you need to know about the effectiveness, safety, and side effects of the three COVID-19 vaccines, along with frequently asked questions about booster shots.

How Do the Different COVID-19 Vaccines Work?

Unlike traditional vaccines such as the flu vaccine, the Pfizer, Moderna, and J&J COVID-19 vaccines don’t use a weakened or inactivated form of the infection-causing virus to trigger an immune response.

The Pfizer and Moderna vaccines rely on a genetic molecule called messenger RNA (mRNA). According to the CDC, instructions contained within the mRNA cause cells in the body to produce a harmless piece of what is called a spike protein — resembling the spike protein on the surface of the novel coronavirus.

The cells display the protein piece on their surfaces, the CDC explains, prompting an immune response that includes the production of antibodies. These remain in the blood ready to launch a defense against any future coronavirus infection.

Unlike the Pfizer and Moderna vaccines, the J&J vaccine is a viral vector vaccine. Instead of mRNA, it uses a genetically altered and weakened adenovirus, a category of virus that includes the type behind the common cold.

How Well Do COVID-19 Vaccines Work?

It’s getting harder to answer this question as new variants arise, says William Werbel, MD, assistant professor of medicine and infectious disease specialist at Johns Hopkins Medicine in Baltimore.

“In the beginning, for both the Pfizer and Moderna original two-dose series, protection against symptomatic infection was about 95 percent and protection against severe disease, hospitalization, and death was nearly 100 percent,” says Dr. Werbel. This was found in the clinical trial data with what is called the “ancestral” variants — the early variants that came before delta and omicron, he adds.

The J&J vaccine was 85 percent effective at preventing severe disease and (in U.S. trials) 72 percent effective at preventing illness overall.

The rise of delta last spring and summer raised concerns about how much protection the COVID-19 vaccines could offer. “With the delta wave, we saw the vaccine’s effectiveness against the variant go down pretty significantly. Once you got three, four, five months out from vaccination, we started seeing the effectiveness against infection go from 90 to 80 to 70 and sometimes even 60 percent,” says Werbel.

Even so, “The vaccines were still very good protection against severe disease — close to 90 percent, which is great,” he adds.

With the heavily mutated omicron variant, two doses of the Pfizer and Moderna vaccines are just 30 to 50 percent effective against infection, says Werbel. Numbers for the J&J vaccine appear to be even lower.

Although the overall effectiveness of the Pfizer and Moderna vaccines has gone down, resulting in more breakthrough cases, immunization still protects against severe disease. Research from South Africa, reported in December 2021 in The New England Journal of Medicine, found that the original series of the Pfizer vaccine offered 70 percent protection against hospitalization due to omicron.

It also appears that omicron causes less severe disease than the delta variant, according to data shared by the CDC’s Dr. Walensky, in a White House press briefing in January 2022. Walensky referenced a preprint study (not yet peer reviewed) that found that patients with omicron had 53 percent less risk of hospitalization, 74 percent less risk of ICU admission, and 91 percent less risk of death when compared to people infected with delta.

Yet given how contagious omicron is and the spike in COVID-19 cases in the United States, vaccines and boosters remain the best way to stay healthy and protect others who may be particularly vulnerable to severe disease.

How Much Extra Protection Can You Get From COVID-19 Boosters?

“If you have a normally functioning immune system, the addition of the booster restores protection quite a bit — again, closer to 85 to 90 percent against severe infection. It’s not exactly where we started, but it restores a lot of protection, even against omicron,” says Werbel.

He explains, “The booster not only improves the amount of antibodies you have, but also the breadth of your response. It also makes your immune response a little bit smarter, so it can better recognize the harmful cells.”

He adds this qualifier: “It is probable, based on some very early data from the United Kingdom, that after two or three months, the effectiveness of the booster may go down again. Again, this is the natural way of vaccines, but it should protect against severe illness for quite some time, for the average person.”

The CDC guidelines for booster eligibility vary depending on how old you are and what type of vaccine you received. The current recommendations are as follows:

Pfizer-BioNTech If it’s been at least five months since you got your second shot and you are 18 years old or older, you can get a booster of any of the available COVID-19 vaccines. Children age 12 to 17 can only get a Pfizer booster.

Moderna If it’s been at least five months since you got your second shot and you are 18 years old or older, you can get a booster of any of the available COVID-19 vaccines.

J&J If it’s been at least two months since you had the vaccine and you are 18 or older, you can get a booster of any of the available vaccines.

What Are the Current Guidelines Around the J&J Vaccine?

The data has been pretty clear that the J&J vaccine has much lower efficacy against infections, partly because it is a one-shot series rather than two, says Werbel.

In December 2021, the CDC stated, “In most situations, the Pfizer-BioNTech or Moderna COVID-19 vaccines are preferred over the J&J vaccine.”

That’s partly because mRNA vaccines are more effective at providing protection against infection and hospitalization, and partly because of a rare side effect with the J&J vaccine, says Werbel. “It’s a severe clotting disorder, particularly in younger to middle-aged women, that has caused some severe outcomes,” he says.

“You can still get the J&J vaccine, and for some people that might be a good option if you’ve had a severe reaction after one of the mRNA vaccines,” says Werbel. If the J&J vaccine is your only option due to limited access to one of the other COVID-19 vaccines, it’s certainly better than no vaccine, he adds.

If I Already Had COVID-19, Do I Need a Vaccine?

“At least in the omicron era, I think it’s become very clear that prior infection is not protective in terms of getting the omicron variant of COVID-19 — it basically blows right through that,” says Werbel.

A report published in December 2021 from the Imperial College London estimated that the protection against reinfection by omicron may be as low as 19 percent.

It would be incorrect to think that any natural immunity achieved by being infected and recovering is the best way to protect yourself from COVID-19, Werbel says. “There is some reduction in the likelihood that you will die as a result of COVID-19, and that’s great — but you really need the vaccine and booster to give you the protection against infection and severe illness,” he says.

What About Children and Vaccines and Boosters?

The current guidance is that children 12 and over can get a Pfizer booster if it’s been five months since they had the original vaccine series.

It’s likely that children age 5 and up will also be eligible for boosters sometime soon, Werbel says.

It's also likely that vaccines will eventually be authorized for children below age 5, he says, with clinical trials underway. “Right now,” Werbel says, “some of that research is focused on getting the right dose to elicit the ideal immune response from small children.”

While children generally experience mild or no symptoms from COVID-19, “Protecting kids not only protects their individual health, it also keeps day cares open, schools open, and keeps grandma from getting infected. There are a lot of public health benefits from kids getting vaccinated,” says Werbel.

How Do We Know the COVID-19 Vaccines Are Safe?

As of December 2021, more than 469 million doses of the COVID-19 vaccine have been administered in the United States, and the vaccines has been shown to be safe. The safety monitoring of the COVID-19 vaccine is the most intensive in U.S history, according to the CDC.

Although many people experience side effects after getting the vaccine such as a sore arm, fever, headache, and fatigue, those typically go away in a day or two.

Reports of rare complications include the following four issues:

Anaphylaxis This severe allergic reaction after COVID-19 vaccination has occurred in approximately 5 people per million immunized in the United States. This kind of reaction can occur after any vaccination, and people at vaccination sites are trained and equipped to immediately offer treatment.

The CDC recommends that people who have a history of anaphylaxis due to any cause remain under medical observation for 30 minutes after vaccination; everyone else, 15 minutes.

Thrombosis With Thrombocytopenia Syndrome (TTS) Associated with the J&J vaccine, this blood-clotting and low-platelet disorder has occurred but is rare. Of the 17.2 million people who received the J&J vaccine as of December 2021, there were 57 confirmed reports of people who later developed TTS. The CDC has identified nine deaths caused by or directly attributed to TTS following the J&J vaccine.

Women under 50 may be particularly affected by this increased risk.

Based on available data, there is no increased risk for TTS with Pfizer or Moderna COVID-19 vaccines.

Warning signs of TTS might be a very severe headache (not the dull headache that happens within a day or two of getting the vaccine), severe abdominal pain, new severe leg pain, or difficulty breathing, says Adrienne Shapiro, MD, PhD, assistant professor of medicine and infectious disease physician at the University of Washington Medicine in Seattle.

Guillain-Barré Syndrome (GBS) Reported in some people who received the J&J vaccine, GBS is a rare disorder in which the immune system damages nerve cells and causes muscle weakness and, in some cases, paralysis. Although most people fully recover from GBS, permanent nerve damage can occur.

Of the 17.2 million J&J vaccine doses administered as of November 2021, there were 283 preliminary reports of GBS. These cases have largely been reported about two weeks after vaccination and mostly in men, many of whom are 50 years and older.

Myocarditis and Pericarditis Inflammation of the heart muscle or the heart’s outer lining, respectively, after COVID-19 vaccination is rare. As of December 2021 there were 1,124 confirmed reports of myocarditis or pericarditis among people ages 30 and younger who received a COVID-19 vaccine, mostly after Pfizer or Moderna vaccination and particularly involving adolescent boys and young adults.

Most patients who were treated for these conditions recovered quickly, says the CDC.

Is It Okay to Take Pain Relievers Before or After Getting Vaccinated?

It’s not recommended that people take any pain relievers such as aspirin, acetaminophen (Tylenol), or ibuprofen (Advil, Naproxen) before getting vaccinated, according to the CDC. The agency also advises against taking an antihistamine before the vaccine to try to prevent an allergic reaction. Per the agency, if you take any of those medications regularly for other health reasons, you should continue taking them, even before the vaccination.

“There is a theoretical chance that taking these medications before getting a vaccine can dampen the body’s ability to have an effective immune response to the vaccine,” says Dr. Shapiro.

“However, if someone experiences common reaction symptoms after getting the vaccine, like arm soreness, headache, or low-grade fevers, it is fine to take pain relievers to treat these symptoms,” she adds.

Is It Safe for Me to Take the COVID-19 Vaccine if I’m Pregnant?

The American College of Obstetricians and Gynecologists recommends that pregnant and lactating women receive a COVID-19 vaccine.

Studies that include pregnant women indicate that although the absolute risk for severe COVID-19 is low, there is an increased risk of severe illness, ICU admission, a need for a ventilator, and death in pregnant women with symptomatic COVID-19 compared with symptomatic nonpregnant women.

Pregnant women who also have conditions such as obesity and diabetes may be at a higher risk of severe illness due to COVID-19 compared with women who have those conditions who are not pregnant.

There is also evidence that vaccinated women pass COVID-19 antibodies to breastfeeding babies, according to a study published in January 2022 in Obstetrics & Gynecology. Researchers found evidence of antibodies in both breast milk and the stools of breastfed infants.

Is It Safe for Me to Take the COVID-19 Vaccine if I Have an Underlying Health Condition?

Yes — and in fact, health experts generally recommend that you do, because you are at a higher risk of developing severe COVID-19 should you become infected.

According to the CDC, health conditions that raise your risk of developing severe COVID-19 include:

Cancer
Chronic kidney disease
Chronic liver disease
Chronic lung diseases, including asthma (moderate to severe), chronic obstructive pulmonary disease, and cystic fibrosis
Dementia or other neurological conditions
Diabetes (type 1 or type 2)
Down syndrome
Heart conditions
HIV infection
Immunocompromised state, including a weakened immune system caused by prolonged use of corticosteroids or other immune-weakening medicines (Note: Some individuals with autoimmune diseases such as rheumatoid arthritis who take medications that suppress their immune systems may qualify.)
Mental health conditions, including depression and schizophrenia
Overweight and obesity
Pregnancy
Sickle cell disease or thalassemia
Smoking, currently or formerly
Solid organ or blood-stem-cell transplant
Stroke or cerebrovascular disease, which affects blood flow to the brain
Substance use disorders
Tuberculosis

If you have any questions or concerns about your individual risk, discuss them with your healthcare provider.

What Do We Know About Vaccine Effectiveness in People Who Are Immunocompromised?

Moderately or severely immunocompromised people remain at high risk for becoming very sick from COVID-19 despite getting two-dose vaccines and boosters, says Werbel. “We are seeing immunocompromised people getting critical illness and even dying who have gotten three doses of the vaccine,” he says.

“This is usually in the context of people not developing antibodies against the virus despite being vaccinated due to the medicines that they take every day for immune suppression or the cancer they had, or something like that,” says Werbel.

Some moderate to severely immunocompromised individuals should get a fourth dose of a Pfizer or Moderna vaccine five or more months after getting their third shot, according to the CDC. If you aren’t sure about your status, talk with your pharmacist or health care provider.

Is It Safe to Get a COVID-19 Shot or a Booster Shot and a Flu Shot at the Same Time?

“In clinical trials, it’s been shown that you can receive your flu shot in very close proximity to the COVID-19 vaccine, and so this is not something people need to be concerned about. We do not think there is any interaction between the two vaccines,” says Anna P. Durbin, MD, a professor at the Bloomberg School of Public Health and the School of Medicine at Johns Hopkins Medicine in Baltimore, Maryland.

It’s also safe to get a COVID-19 booster shot at the same time as your flu shot.

The CDC does recommend that you try to follow the recommended schedule for either vaccine, which means ideally getting a flu shot by the end of October (though it’s never too late) and getting any recommended COVID-19 shot or booster as soon as you are eligible.

I Don’t Trust Vaccines. Isn’t It Better to Let My Immune System Do Its job Naturally?

Vaccines are the best way to protect yourself and those around you. Previous infection hasn’t provided a strong defense against COVID-19, says Werbel.

A CDC study published in November 2021 in Morbidity and Mortality Weekly Report examined adults hospitalized for COVID-like sickness between January and September 2021. Investigators found that the chance of these adults testing positive for COVID-19 was more than five times higher in unvaccinated people who had COVID-19 in the past than it was for people who had not had COVID-19 but who had been vaccinated.

Another reason that getting COVID-19 isn’t the best path to herd immunity is the possibility of symptoms lingering for weeks, months, or longer — so-called “long COVID.” According to research from the University of Michigan, more than 40 percent of COVID-19 survivors around the world have or had long-term issues, even after recovering from the acute stage of infection.

These symptoms can include difficulty breathing or shortness of breath, fatigue, brain fog, headache, joint and muscle pain, and sleep problems, according to the CDC.

Where Can I Get a COVID-19 Vaccine or Booster?

Many state and local health departments list COVID-19 vaccination sites online. The Washington State Department of Health, for instance, maintains a county-by-county guide to clinics, hospitals, pharmacies, and other locations that offer COVID-19 vaccines. Some places schedule appointments; others are first come, first served.

The website Vaccines.gov is an invaluable resource that allows you to enter your ZIP code and preferred type of COVID-19 vaccine to find immunization sites nearby. You can also text your ZIP code to 438829 or call 800-232-0233 to find places that offer the shot near you.