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Quick guide: COVID-19 vaccines in use and how they work

gloved hands holding syringe, giving injection into a patient's arm
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Editor's note: This article was last updated at 9:40am on April 9. The original article was posted on Jan. 11. 

Dozens of coronavirus vaccines entered clinical trials during 2020, and now, a handful have been authorized for emergency use in various countries — meaning the shots can be administered to the public while their developers continue to collect data on their safety and efficacy. Should they meet all the necessary criteria, these vaccines could be fully approved in the future, and in some places, they already have been.

Here's a quick guide to the COVID-19 vaccines now in use around the world.

Related: 20 of the worst epidemics and pandemics in history

Pfizer-BioNTech vaccine

The vaccine developed by Pfizer and German biotechnology company BioNTech is 95% effective at preventing COVID-19, a large study found. The vaccine is administered in two doses, given three weeks apart, and it must be stored at minus 94 degrees Fahrenheit (minus 70 degrees Celsius). On Dec. 11, 2020, the Pfizer-BioNTech vaccine became the first COVID-19 vaccine to be authorized for emergency use by the U.S. Food and Drug Administration (FDA). 

Several other countries have also authorized the vaccine for emergency use, including the U.K., Argentina, Chile and Singapore, and the European Union followed suit on Dec. 21, The New York Times reported. Bahrain, Canada, Saudi Arabia and Switzerland have fully approved the vaccine. On Jan. 25, Australia gave the vaccine "provisional approval" for use in people aged 16 and over, The Associated Press reported

On March 31, Pfizer announced that the vaccine was 100% effective in preventing COVID-19 among children ages 12 to 15 in a late-stage clinical trial.

The shot uses a molecule called mRNA as its base. A molecular cousin of DNA, mRNA contains instructions to build specific proteins; in this case, the mRNA in the vaccine codes for the coronavirus spike protein, a structure that sticks off the virus's surface and is used to infect human cells. Once inside the body, the vaccine instructs human cells to build this protein, and the immune system learns to recognize and attack it.

Moderna/NIAID vaccine

The vaccine developed by U.S. biotech company Moderna and the National Institute of Allergy and Infectious Diseases (NIAID) also uses mRNA as its base and is estimated to be 94.5% effective at preventing COVID-19. Like the Pfizer-BioNTech vaccine, it's delivered in two doses, but the doses are given four weeks apart, rather than three. Another difference is that the Moderna vaccine can be stored at at minus 4 F (minus 20 C), rather than requiring deep-freezing like the Pfizer shots.

The FDA authorized the Moderna vaccine for emergency use on Dec. 18, and Israel and the European Medicines Agency, an agency of the European Union, both authorized the shots for emergency use in January, according to the Times. Canada fully approved the vaccine on Dec. 23. 

Oxford-AstraZeneca vaccine

The vaccine developed by Oxford University and pharmaceutical company AstraZeneca is estimated to be about 70% effective at preventing COVID-19 — that said, in clinical trials, adjusting the dose seemed to boost this efficacy. 

In people given two full-size doses, spaced 28 days apart, the vaccine was about 62% effective; in those given a half-dose followed by a full dose, the vaccine was 90% effective, according to early analyses. However, clinical trial participants who got half-doses did so by mistake, and some scientists have questioned whether those early results are representative.   

The U.K. and Argentina authorized the Oxford-AstraZeneca vaccine for emergency use in late December, and India and Mexico authorized the vaccine for emergency use in January, according to the Times. The EU's European Medicines Agency authorized the vaccine for use in adults on Jan. 29, The Associated Press reported.  Canada approved the vaccine on Feb. 26, CTV News reported.

Dozens of countries temporarily paused their use of the shots when reports of blood clots and abnormal bleeding surfaced among some vaccinated people. A subsequent review by the EU's European Medicines Agency concluded that the shots don't increase the overall incidence of blood clots, but the committee could not rule out a link to some rare clotting disorders, Live Science reported

The committee then launched a deeper investigation into these rarer clotting events and found that they are linked to the vaccine, although exactly how the shots trigger clotting is unknown. Blood clots and low platelet counts should be listed as possible side effects of the vaccine, as these effects may emerge in roughly "one in 100,000" vaccinated people, the EMA stated; the exact rate at which vaccinated people develop the clots is currently unknown. 

Related: AstraZeneca COVID-19 vaccine causes very rare blood clots, EU regulator says

The shots are still under review in the U.S., and American officials recently concluded that the vaccine is 76% effective in preventing symptomatic COVID-19, The Associated Press reported.

The shots contain a weakened version of adenovirus, a common cold virus that naturally infects chimpanzees. Scientists modified the virus so it cannot replicate in human cells and then added genes that code for the coronavirus spike protein. Inside the body, the vaccine enters cells and delivers these spike protein genes, which the cells use to build the spike protein itself. The presence of spike proteins triggers an immune response. 

White box of Pfizer-BioNTech vaccines being placed in a freezer by two gloved scientists in lab coats

Pfizer-BioNTech vaccines being placed in a freezer at the University Hospital in Brussels, Belgium on December 26, 2020. (Image credit: Nicolas Maeterlinck - Pool#OM/Getty Images)

Johnson & Johnson / Janssen

The vaccine developed by Johnson & Johnson's Janssen showed a 72% overall efficacy rate in the U.S. and 64% in South Africa, where a highly-transmissible coronavirus variant is causing most new cases, according to an FDA analysis conducted in February, the Times reported

In terms of preventing severe disease, the single-shot vaccine showed 86% efficacy in the U.S. and 82% in South Africa; and in both locations, the vaccine showed 100% efficacy against hospitalization and death, Live Science reported. This means that no one given the vaccine in the clinical trials had died from COVID-19 or required hospitalization for the disease when assessed at 28 days post-vaccination.  

On Feb. 25, following the FDA analysis, Bahrain became the first country to authorize the vaccine for emergency use. The U.S. cleared the vaccine for emergency use on Feb. 27.

Similar to the vaccine developed by the University of Oxford and AstraZeneca, the Johnson & Johnson shot contains a weakened version of a common cold virus, known as an adenovirus, Live Science previously reported. Scientists genetically altered the adenovirus, named ad26, so it can no longer infect human cells and then added genes that code for the coronavirus spike protein

Once inside the body, the vaccine trains the immune system to recognize the spike protein and target the coronavirus for destruction. Janssen used a similar strategy to develop its existing Ebola vaccine. The shot can be stored for months at normal refrigerator temperatures.

Sinopharm (Beijing Institute of Biological Products)

Sinopharm, the state-owned China National Pharmaceutical Group, and the Beijing Institute of Biological Products developed a vaccine from an inactivated coronavirus, meaning a modified version of SARS-CoV-2 that cannot replicate. In late December, Sinopharm announced that the vaccine, called BBIBP-CorV, is more than 79% effective, according to preliminary data from late-stage clinical trials — that said, the company has not published said data.

In the summer of 2020, China gave Sinopharm authorization to vaccinate construction workers, diplomats and students with one of its two COVID-19 vaccine candidates, including BBIBP-CorV, Live Science previously reported. Nearly 1 million people had received the vaccines by November, according to the company's chairman.    

The United Arab Emirates authorized BBIBP-CorV for emergency use in September and then fully approved the vaccine in December. Bahrain and China also fully approved the vaccine in December, and Egypt authorized it for emergency use in January 2021, the Times reported. The vaccine is given in two doses spaced three weeks apart.

Sinopharm (Wuhan Institute of Biological Products)

Sinopharm's second vaccine candidate, developed by the Wuhan Institute of Biological Products, also uses an inactivated coronavirus as its base. The vaccine has been authorized for emergency use in China and the U.A.E., but little is known about its efficacy. 

CanSino-Beijing vaccine

CanSino Biologics, in collaboration with the Beijing Institute of Biotechnology, developed a COVID-19 vaccine using a weakened adenovirus, but one that naturally infects humans, not chimpanzees. Late-stage clinical trials with the vaccine are still ongoing, and its efficacy is not yet known. The shot is given in a single dose.

In June 2020, the CanSino vaccine was given approval to be used by China's military, according to Reuters

CoronaVac by Sinovac

The Chinese company Sinovac Biotech developed a vaccine from an inactivated version of SARS-CoV-2. The vaccine, called CoronaVac, is given in two doses 14 days apart, Live Science previously reported. China authorized the vaccine for emergency use in July.

Estimates vary as to how well the vaccine protects against COVID-19, and an official estimate has not yet been issued. One clinical trial in Brazil suggested that the vaccine was about 78% effective in one small subgroup of patients, but across all people, the efficacy may be closer to 63%, according to Estadão, a Brazilian news outlet. 

On Jan. 12, researchers from the Butantan Institute, the biologic research center where the trails were conducted, announced that the efficacy rate is 50.4%, overall, based on the latest data, BBC News reported

Bharat Biotech vaccine

The Indian company Bharat Biotech, along with the Indian Council of Medical Research and the National Institute of Virology developed a vaccine from an inactivated coronavirus, called Covaxin, the Times reported. The vaccine is given in two doses, spaced four weeks apart, and has been authorized for emergency use in India. It's efficacy has not been publicly reported. 

Sputnik V by Gamaleya Research Institute

The Russia Ministry of Health's Gamaleya Research Institute developed a coronavirus vaccine candidate called Sputnik V, in reference to the world's first artificial satellite, launched during the space race, Live Science previously reported. The vaccine contains two common cold viruses, or adenoviruses, that have been modified so they don't replicate in humans; the modified viruses also contain genes that code for the coronavirus's spike protein.

In November, Russia announced that the vaccine is more than 91.4% effective in preventing COVID-19, according to early data from clinical trials. On Feb. 2, scientists reported that the vaccine has 91.6% efficacy, based on preliminary data from a late-stage trial published in The Lancet.

Russia began offering the vaccine to its citizens as part of a mass vaccination campaign in November 2020, according to The New York Times. Russia had already approved the vaccine for limited use in August, when very little data from human trials was available, Live Science reported.

Since November, Belarus, Argentina and Serbia have also authorized the vaccine for emergency use, the Times reported. On Jan. 15, Paraguay also authorized the vaccine for emergency use, Reuters reported

Vector Institute vaccine

In October 2020, Russia granted "regulatory approval" to a second vaccine, one developed by the Vector Institute, a Russian biological research center, according to CNBC. The vaccine contains coronavirus peptides, which are small portions of proteins found in the virus. Like Sputnik V, the vaccine earned approval in Russia before large-scale clinical trials had been conducted. As of now, its efficacy is still unknown. 

Originally published on Live Science. 

Nicoletta Lanese is a staff writer for Live Science covering health and medicine, along with an assortment of biology, animal, environment and climate stories. She holds degrees in neuroscience and dance from the University of Florida and a graduate certificate in science communication from the University of California, Santa Cruz. Her work has appeared in The Scientist Magazine, Science News, The San Jose Mercury News and Mongabay, among other outlets.