Coronavirus Vaccines 101: Ingredients, Safety and Side Effects

The U.S. kicked off an an unprecedented inoculation campaign this week with a COVID-19 vaccine from Pfizer and its German partner BioNTech. Another vaccine from Moderna, which was co-developed with the National Institutes of Health, moved closer to joining the fight against the pandemic with the endorsement Thursday from a panel of experts.

The move paves the way for a final decision by the Food and Drug Administration on emergency use, with shipments expected to begin this weekend if Moderna is granted approval.

A second vaccine is urgently needed as coronavirus infections, hospitalizations and deaths in the U.S. climb to new highs. The few available doses are mostly going into the arms of health-care workers and nursing home residents. How well initial vaccinations go will help reassure a wary public when it's their turn sometime next year.

Here's what we know so far about the two vaccine's, who's next in line and where they will be administered:


The Centers for Disease Control and Prevention decided early this month that health care workers and nursing home residents would be first in line for the initial, limited supply.

A federal panel of vaccination experts will convene an emergency meeting this weekend to deliberate on who is next. The panelists are leaning toward putting “essential workers” first because bus drivers, grocery store clerks and similar employees can’t work from home. They are the people getting infected most often, and where concerns about racial inequities in risk are most apparent.

But other experts say people age 65 and older should be next, along with people with certain medical conditions. Those are the people who are dying at the highest rates, they say.

No matter what the committee decides, the decision will ultimately be up to the states. In California, for example, teachers and food and agriculture workers were among those recommended to get the next round of vaccines.

More vaccines are in the pipeline, and officials have said they want to have shots widely available to everyone else before the middle of next year.

To find out when it is your turn, watch for notices from your state.


The FDA’s Dr. Peter Marks said Saturday that pregnant women should consult their doctor before taking the Pfizer vaccine, given that pregnant women were not part of the clinical trials for the emergency use authorization. “The provider and individual can make a decision about whether the benefits outweigh the risks,” said Marks. “[Taking the vaccine is] not something that we’re recommending at this time. That’s something that we’re leaving up to the individual.”

The Pfizer vaccine is authorized for people 16 and older. Testing began in October in children as young as 12 and is expected to take several more months. The FDA will have to decide when there’s enough data to allow emergency use in this age group. Depending on the results, younger children may be enrolled for study as well.

Moderna began enrolling study participants ages 12 to 17 this month, and will track them for a year. Testing in children younger than 12 is expected to start in early 2021.

It is uncertain if the results on younger children will come in time for vaccinations to begin before the next school year.

Pregnant women have been excluded from large-scale clinical trials, so there is limited data on the safety of COVID vaccines for people who are pregnant.

However, based on how mRNA vaccines work, experts believe they are unlikely to pose a risk for pregnant women since they don't contain the live virus that causes COVID-19 and therefore cannot give someone COVID-19.

The CDC panel said pregnant women could get the shot, but said they might want to consult with their doctor first.


Secretary of Health and Human Services Alex Azar said on Monday the United States has purchased another 100 million doses of Moderna’s COVID-19 vaccine. Azar also said that he expects to be able to vaccinate every American who wants it by the end of the second quarter in 2021.

The U.S. currently has at least 6.4 million Pfizer doses and has shipped around 3 million doses to 636 locations around the country for the first round of vaccinations. Since the Pfizer vaccine requires two doses, the government is holding back the second shots to ensure people can get them three weeks after the first dose.

The current shipment, however, may be able to inoculate more people than initially thought. That's because hospitals discovered the vials they were told held five doses actually yields six and sometimes seven doses, potentially adding to the country's supply.

An additional 500,000 doses are being held in reserve for any emergencies, according to Army Gen. Gustave Perna of Operation Warp Speed, the government effort to develop and distribute COVID-19 vaccines.

Each state was granted an amount based on the population of people 18 and older. To see how many doses each state received in the first round of distributions, click here.

Once approved, the second vaccine from Moderna and the National Institutes will ship out 6 million doses of that shot in the first wave to 3,285 locations across the U.S. Moderna's vaccine also requires two shots, but the second shot comes four weeks after the first.

Federal officials say they expect to have enough vaccines to give 20 million people their first doses by the end of the year — meaning they’d have around 40 million shots available — and 30 million more in January.

The government’s Operation Warp Speed program has orders for 200 million doses of Moderna’s vaccine. That’s on top of 100 million doses of the Pfizer-BioNTech vaccine. Officials are negotiating to purchase more doses of that vaccine and there are more vaccines in the pipeline.


The simple answer is that they lose potency and effectiveness if the temperatures drop below the required storage temperatures.

Pfizer's vaccine must be shipped at minus 70 degrees Celsius ( minus 94 F) and will stay safe and potent for up to six months if stored in the same ultra cold temperatures. Once transferred to a refrigerator, which maintains temperatures at at 2 to 8 degrees Celsius (26-46 F), it must be administered within five days. Once it has thawed, the vials cannot be refrozen, Pfizer said.

Moderna's vaccine has to be shipped at minus 20 degrees Celsius ( minus 4 F) and can then be stored at that temperature for up to six months. Once thawed and kept in a refrigerator between 2 and 8 degrees Celsius (36 to 46 F) it is good for up to 30 days.


States are signing up pharmacies, health clinics and doctor’s offices to give the shots. Local health departments will also probably run mass vaccination clinics.

The government struck a deal in October with CVS and Walgreens to administer the coronavirus vaccinations to residents and staff at long-term care facilities first, and eventually the general public. 

CVS Health CEO Larry Merlo told NBC's TODAY show that patients will have to schedule a vaccine appointment via the CVS app,  unlike being able to just walk into a CVS and get a seasonal flu shot.

Once doses are widely available, people will be able to use an existing government website,, to find COVID-19 shots. The website is already used to find vaccines for the flu and other diseases.

COVID-19 shots are likely to be limited for awhile, and which type people get will probably depend on what’s available.


It should be free. The government is paying for the vaccine itself. And you shouldn’t be charged a copay or other fee to get it.

Even if a provider charges a fee for giving the shot, the cost will be covered by private and government insurance, based on a set reimbursement fee. If you don’t have insurance, providers can tap a government fund to cover costs.


It depends on the brand of vaccine. Pfizer’s is three weeks later, and Moderna’s is four weeks.

Each state's strategy on how they will remind people to return for their second vaccine shot varies. Some have said they will provide vaccination record cards as a reminder to return for a second shot. Others plan on sending reminder texts, calls or letters in the mail, depending on the location.

Shots will be recorded in state and local vaccine registries that already keep track of other vaccinations. COVID-19 vaccines can’t be mixed and matched, so if a second dose is needed, providers will be checking to make sure you get the right one.

One concern, however, is that because the U.S. lacks a national database, if people leave their state before their scheduled second shot, health officials won't have access to previous immunization information. Current efforts to create a national vaccine registry are raising privacy concerns from some local officials and a number of states have refused to sign the CDC's data use agreement.

Not all vaccines in development require two shots. Johnson & Johnson is testing a single-dose vaccine.


Both the Moderna and Pfizer/BioNTech COVID-19 vaccines were developed using mRNA—short for messenger RNA— technology. Here is how they work.

The vaccines from Pfizer and Moderna use the same groundbreaking technology to protect you from infection.

Most traditional vaccines use dead or weakened virus, but both of the new vaccines use snippets of COVID-19's genetic code, called messenger RNA or mRN, to train the immune system to detect and fight the virus. Since they aren’t made with the coronavirus itself, there is no chance anyone could catch it from the shots.

Pfizer's vaccine became the first RNA vaccine to be approved for use in humans.


Pfizer's new COVID-19 vaccine contains the genetic material known as mRNA, as well as three categories of ingredients: fats, salts and sugar.

Pfizer made a list of its ingredients available in a letter to the FDA published here. They are: mRNA, lipids ((4-hydroxybutyl)azanediyl)bis(hexane-6,1-diyl)bis(2-hexyldecanoate), 2 [(polyethylene glycol)-2000]-N,N-ditetradecylacetamide, 1,2-Distearoyl-sn-glycero-3-phosphocholine, and cholesterol), potassium chloride, monobasic potassium phosphate, sodium chloride, dibasic sodium phosphate dihydrate, and sucrose.

While the two vaccines use the same mRNA technology, they’re not identical, cautioned Moderna chief medical officer Dr. Tal Zaks. In particular, some of the lipids, or fats, used to coat the two vaccines are different.

Moderna's ingredients can be found at the FDA site here. They are: mRNA, lipids (SM-102, 1,2-dimyristoyl-rac-glycero3-methoxypolyethylene glycol-2000 [PEG2000-DMG], cholesterol, and 1,2-distearoyl-snglycero-3-phosphocholine [DSPC]), tromethamine, tromethamine hydrochloride, acetic acid, sodium acetate, and sucrose.


Moderna has announced that it has started to study the impact of its coronavirus vaccine on 3,000 adolescents.

The Pfizer vaccine, called BNT162b2, is 95% effective, is safe and also protects older people most at risk of dying, according to results of its late-stage trial announced in November.

A final data analysis of the more than 43,000 participants found the vaccine was highly effective against the virus 28 days after the first dose, and its efficacy was consistent across all ages, races and ethnicities, the drugmakers said. Additionally, the elderly, who are seen as at high risk of severe illness from COVID-19, saw vaccine effectiveness of more than 94%, they added.

Moderna’s shot, dubbed mRNA-1273, showed similarly strong effectiveness, providing 94% protection against COVID-19 in the company’s ongoing study of 30,000 people. A final analysis of its phase three clinical trial found the vaccine also found it to be safe and appeared to fend off severe disease. 


As the nation gets ready to start receiving vaccinations for COVID-19, it's important to understand the side effects. Here is what doctors are saying people can expect.

Both experimental vaccines have been tested in tens of thousands of volunteers so far, and serious side effects have not been reported. Health officials will be monitoring for side effects as more people get vaccinated, as well as for any potential longer-term issues.

Getting either the Pfizer-BioNTech shot or the Moderna version can cause some temporary discomfort, just like many vaccines do. In addition to a sore arm, people can experience a fever and some flu-like symptoms — fatigue, aches, chills, headache. They last about a one to three days, sometimes bad enough that recipients miss work, and are more common after the second dose and in younger people.

These reactions are a sign that the immune system is revving up.

FDA staff recommends monitoring people who get Pfizer or Moderna's vaccine shots for possible cases of Bell's palsy, saying it's not necessarily a side effect but worth watching out for after a handful of trial participants got the condition, which causes a temporary facial paralysis.

There were four reported cases of Bell's palsy among Moderna's more than 30,000 clinical trial participants. Three of the participants who got Bell's palsy received the vaccine and one person who got a placebo shot. Pfizer's trial similarly had four reported cases of Bell's palsy out of some 43,000 participants. All four Bell's palsy cases in Pfizer's trial got the vaccine and not the placebo.

The FDA review staff said there wasn't enough data to tie the cases directly to the shots, but it warranted close scrutiny.


British officials are investigating reports that two people who received the Pfizer vaccine had allergic reactions. Medical experts say that while reactions are rare, they aren’t unheard of for vaccines of any kind and are usually short-lived.

Vaccines can sometimes cause allergic reactions, but they are usually rare and short-lived. The first allergic reports from England were in people with a history of serious allergies, and British authorities warned those with severe prior experiences to hold off vaccination as they determine what ingredient might be a problem.

A case also occurred in an Alaskan health-care workers who took Pfizer's vaccine earlier this week. Health officials said the middle-aged woman had no history of allergies and had never experienced anaphylaxis, a severe and life-threatening allergic reaction. Pfizer said in a statement that the company is "working with local health authorities to assess" the reaction, and will "closely monitor all reports suggestive of serious allergic reactions following vaccination and update labeling language if needed."

The CDC advises people who have had severe allergic reactions to other vaccines or drugs to talk to their doctors first before getting the vaccine. Those who've had any kind of severe allergic reaction in the past should be watched for 30 minutes after vaccination. Others should be watched for 15 minutes.

Instructions for the Pfizer-BioNTech shot say to avoid it if you’re severely allergic to one of its ingredients or had a severe reaction to a prior dose. Health workers can go over the ingredient list.

The FDA found no severe allergic reactions in Moderna’s data but flagged a slightly higher rate of less serious side effects — rash, hives, itching — among participants who got the vaccine, compared with those receiving a dummy shot.


The first COVID-19 vaccinations could begin within days after the Food and Drug Administration grants emergency use authorization of a vaccine, but that doesn't mean life will go back to normal right away. Millions of vials must be safely and securely shipped across the country, in a logistical puzzle that could take months to complete.

No. For a couple reasons, masks and social distancing will still be recommended for some time after people are vaccinated.

To start, the effect of vaccinations generally aren’t immediate, and the first vaccines by Pfizer and Moderna require two shots.

People are expected to get some level of protection within a couple of weeks after the first shot. But full protection may not happen until a couple weeks after the second shot. And even if they're protected from getting sick, vaccinated people might still be able to get infected and pass the virus on, although it would likely be at a much lower rate.

Once vaccine supplies start ramping up, it will still still take months to get hundreds of millions of shots into people's arms.


Having COVID-19 gives you protection against another infection, but it is not known yet how long that immunity will last. And there’s nothing to indicate that previously infected people shouldn’t get vaccinated, according to the CDC advisory panel.

By vaccinating those with prior infections, “The expectation is for them to be more protected,” said Moncef Slaoui, who’s heading U.S. vaccine development. But studies will be needed to support that, he said.

People with a prior COVID-19 diagnosis weren’t allowed to volunteer for the vaccine tests. But there was no screening to rule out people who might have been previously infected and not known it. So some people with symptom-free infections might have been included. The CDC panel said there was no need to test for those silent infections before getting a vaccine.

For health care workers who had COVID-19, the panel suggests that they might want to let their fellow health workers go ahead of of them since the chance of reinfection is low for the next three months.

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