The U.S. Food and Drug Administration has granted emergency use authorizations for three COVID-19 vaccines so far. We compare the different available shots, and several still in the approval process for use in the United States.
Which vaccines are approved already?
The Pfizer-BioNTech and Moderna vaccines have been available to Americans since December.
A vaccine developed by Janssen Pharmaceuticals, a Johnson & Johnson company, was authorized in late February.
AstraZeneca, working in collaboration with Oxford University, is expected to request authorization from the FDA for their vaccine within a few weeks, and Novavax is expected to follow with its application later in April or May.
What technology does each vaccine use?
Many vaccines use weakened or inactivated versions or components of the disease-causing pathogen to stimulate the body’s immune response. However, the vaccines developed by Pfizer-BioNTech and Moderna take advantage of messenger RNA (mRNA), which instructs cells to produce a protein on the surface of the virus. The immune system recognizes those vaccine-triggered spike proteins as invaders and creates antibodies to block future attacks of the virus that causes COVID-19.
More: How mRNA vaccines work
The Johnson & Johnson vaccine takes the more traditional approach. Adenoviruses are common viruses that typically cause colds or flulike symptoms. The J&J team used a modified adenovirus that can enter cells but can’t replicate inside them or cause illness. The vaccine contains this modified virus that delivers a spike protein, activating the immune system.
The AstraZeneca-Oxford vaccine also uses an adenovirus, in this case a monkey virus that the human immune system doesn't recognize. It delivers the gene that encodes the spike protein to human cells rather than the protein itself.
Novavax's vaccine works by delivering lab-grown spike proteins along with a compound that attracts immune cells to the site of the injection.
Where is each vaccine developed?
Moderna, Pfizer, Johnson & Johnson and Novavax are all US-based companies. The J&J vaccine was developed by researchers at Harvard University and in Leiden, Netherlands. Moderna, based in Cambridge, Massachusetts, developed its vaccine in collaboration with U.S. government scientists. The AstraZeneca vaccine was developed by the University of Oxford and its spin-off company, Vaccitech. BioNTech, a biotechnology company based in Mainz, Germany, partnered with Pfizer to test and produce its vaccine.
How many shots do I need to get?
All the vaccines except the one from Johnson & Johnson require two shots to train the immune system well enough to fight the coronavirus.
After one dose of the Pfizer vaccine, for example, its effectiveness against illness reached up to 52% after 12 days, and up to 95% a week after the second dose, according to a study. That's why it's important for people who have received only one dose to keep practicing the usual precautions, like wearing a mask and maintaining social distance. The Centers for Disease Control and Prevention suggests getting the second shot as close to the recommended dose schedule as possible, but not earlier.
The J&J vaccine is a one-dose vaccine. However, the company is also testing a two-dose regimen, with the two shots given eight weeks apart.
How much do vaccines cost per dose?
Moderna and Pfizer-BioNTech so far have been selling the majority of their doses to high-income countries, including the U.S., Canada and the European Union. These are also the most expensive vaccines.
AstraZeneca-Oxford has the cheapest of the five vaccines. The company has committed not to profit from it while the pandemic lasts.
There have been reports of different prices being paid by governments in different parts of the world.
How much has the U.S. spent on each?
Overall, the U.S. government has made vaccine deals totaling more than $9 billion with multiple private companies, but the deals vary in size.
The pharmaceutical giant Merck will also receive $105 million from the government to help produce the J&J vaccine.
How many doses of each will the U.S. get?
President Joe Biden said in March that the U.S. will have enough vaccines for every adult in the U.S. by the end of May. The main challenge will now be to distribute the doses and to convince hesitant people to take them.
Do we have enough vaccine to go around?
The U.S. has purchased over 1 billion doses of vaccine for a population of 330 million.
According to a ONE's Policy team study, the world’s richest countries have collectively bought 1 billion more doses than their citizens need. The rest of the world has been able to secure only 2.5 billion doses - not enough to vaccinate their populations.
The excess doses purchased by rich countries alone would be sufficient to vaccinate the entire adult population of Africa. In many developing countries, widespread vaccination coverage might not be achieved before 2023, unless countries like the U.S. share their surplus.
Do temperature and storage requirements differ between vaccines?
The mRNA vaccines from Pfizer-BioNTech and Moderna require a complicated cold chain to safely distribute them. During each part of the process, the vaccine boxes must be kept at exactly the right temperature. The Pfizer-BioNTech vaccine must be used within five days after the transfer to refrigerator. Moderna vaccine can last in the refrigerator for a month and the J&J vaccine can be stored in the refrigerator for at least 3 months.
The Novavax and AstraZeneca-Oxford vaccines can be stored in normal refrigerators for up to six months.
What about safety and side effects?
Serious side effects, allergic reactions or adverse incidents stemming from the vaccines are rare, though in clinical trials, mild to moderate side effects were common. The most common complaints were pain at the injection site, fatigue and aching muscles and joints. People with a history of anaphylaxis or severe allergic reactions should inform health professionals before they are vaccinated, and anyone with an allergy to one of the vaccine's ingredients should consult a healthcare provider before getting vaccinated.
The Food and Drug Administration and the Centers for Disease Control and Prevention are recommending the U.S. pause on using the Johnson & Johnson COVID-19 vaccine after reports of blood clots in six of 6.8 million individuals who received the vaccine so far.
All six recipients were women between the ages of 18 and 48, with symptoms occurring 6 to 13 days after vaccination, according to the statement. One woman died and one has been hospitalized in critical condition.
The health agencies released a statement on April 13 recommending the pause “out of an abundance of caution,” saying blood clots still seem to be “extremely rare.” So far, the CDC and FDA are reviewing six reported cases in the U.S.
The type of blood clot reported in individuals who have received the J&J vaccine is called cerebral venous sinus thrombosis (CVST). As of April 12, more than 6.8 million doses of the Johnson & Johnson vaccine have been administered in the U.S. and approximately 9 million more has been shipped out to states, according to the CDC.
The University of Oxford previously reported pausing administration of the vaccine it developed with AstraZeneca in a U.K. study to test the shot in children pending review of rare blood-clotting issues seen in adults after receiving the shot.
Do some vaccines work better than others?
Public health experts emphasize that all the vaccines are effective, particularly at preventing serious disease.
Because trials were conducted differently at different times, effectiveness figures cannot be directly compared.
In its large-scale trial, the Pfizer-BioNTech vaccine was shown to prevent 95% of symptomatic COVID-19, just 1 percentage point more than Moderna's. Both vaccines appear to be equally protective across age groups and racial and ethnic groups.
The J&J vaccine was shown to be 72% effective in moderate to severe disease and 85% effective in preventing the most severe disease.
AstraZeneca-Oxford vaccine was 76% effective at preventing symptomatic COVID-19 two weeks after the second dose and was 100% effective in stopping severe disease and hospitalization in a U.S.-based clinical trial, according to the company.
In a trial in the U.K., two doses of the Novavax vaccine were shown to be nearly 90% effective against symptomatic COVID-19, though just 50% effective in a smaller South African trial.
What do we still need to know?
It remains unclear how long any of these vaccines will continue to protect people. While the small number of early trial participants have maintained levels of protective antibodies for nearly a year, companies will continue to follow volunteers for two years to see if their immunity begins to wane. Periodic booster shots to extend immunity and/or protect against new variants of the virus might be needed.
And although a growing body of research suggests that vaccines will also protect people from passing on the virus – even if they don't have symptoms – that remains to be confirmed.