The three main Covid-19 vaccines are from Pfizer-BioNTech, the University of Oxford and Astra-Zeneca and Moderna. The Pfizer, Oxford and Moderna vaccines each require two doses and you are not fully vaccinated until you have had both shots. But there are many differences between them.
AstraZeneca’s COVID-19 vaccine is 79% effective against symptomatic coronavirus infections and 100% effective in preventing hospitalization and severe disease, according to an interim analysis of the United States phase 3 trial, which included more than 32,000 people.
Even better, AstraZeneca’s vaccine can be stored at normal refrigerator temperatures, meaning it could be key to reaching people in rural and underfunded areas, one of the most pressing issues in the fight against the novel coronavirus.
Although the vaccine hasn’t been authorized for use in the U.S. yet, here’s what we know about it so far, and how it stacks up against Pfizer’s, Moderna’s, and Johnson & Johnson’s vaccine.
Which vaccines require one dose and which require two doses?
They all require two doses at different time intervals. Pfizer suggests the two doses be administered three weeks apart, Moderna suggests four weeks, and AstraZeneca suggests four to 12 weeks apart.
How does the AstraZeneca COVID-19 vaccine work?
AstraZeneca’s vaccine uses adenovirus-vectored technology, a harmless, modified version of a common cold virus that usually spreads among chimpanzees. This altered virus can’t make you sick, but it carries a gene from the novel coronavirus spike protein, the portion of the virus that triggers an immune response. This allows the immune system to manufacture antibodies that work against COVID-19, teaching your body how to respond should you become infected.
In other words, AstraZeneca’s vaccine mimics a COVID-19 infection without its life-threatening side effects, per a release from the company. The reason researchers chose a chimpanzee adenovirus is simple: The modified virus needs to be new to the people being vaccinated — otherwise, the body won’t create those all-important, unique antibodies. Johnson & Johnson’s vaccine also relies on similar adenovirus-vectored tech.
How does AstraZeneca’s vaccine compare to Pfizer’s and Moderna’s?
Storage and distribution
The Moderna and Pfizer options, meanwhile, must be stored at subzero temps until they’re ready to be used, at -4°F and -94°F, respectively. (mRNA tech is relatively more fragile, meaning it must be kept at much lower temperatures to remain effective and stable.)
Higher storage temperatures could make distribution much easier. “A clinic, a nursing home, or even [regional] health departments may not have freezers that can hold things at -94°F,” says Kawsar Talaat, M.D., an infectious disease doctor, vaccine researcher, and assistant professor in the Department of International Health at Johns Hopkins University. Being able to use a typical fridge “allows time for distribution, allows the vaccine time to get to more rural areas, [and allows vaccines] to be kept at a clinic for a longer period of time.”
AstraZeneca’s vaccine is estimated to cost U.S. providers about $4 per dose, per a 2021 BMJ report. Pfizer’s costs about $20 per dose, Moderna’s costs between $25 to $37 per dose, Forbes reports. These amounts will likely fluctuate as time goes on and the vaccines evolve.
All three vaccines’ side effects are similar, including potential injection site pain and flu-like symptoms, including fever, fatigue, headaches, and muscle pain, which are to be expected as your immune system is primed, especially after a second dose.
The two mRNA vaccines have a slight edge in efficacy; both Pfizer and Moderna report being about 95% effective against COVID-19 after the second shot in clinical trials. (For comparison, the annual flu shot is usually between 40 and 60% effective, per the CDC.) They also reduce the risk of severe illness even if you do become infected with SARS-CoV-2.
Interim results from AstraZeneca’s United States trial report that the vaccine is 79% effective against symptomatic COVID-19 infection after two doses spaced four weeks apart; it’s 100% effective against severe illness and hospitalization. Based on a previous study, the AstraZeneca vaccine might become even more effective with doses spaced further than four weeks apart; more research is needed to confirm the significance of this finding, however.
Efficacy against variants
Pfizer’s and Moderna’s trials were done before highly infectious coronavirus variants became widespread; both companies are currently doing more testing to see how their respective vaccines stack up, and whether or not a third booster dose would be necessary for max protection. So far, Pfizer maintains that it has “not seen any evidence that the circulating variants result in a loss of protection,” while Moderna says initial data shows that its vaccine “provides neutralizing activity against variants of concern.”
South African research on the AstraZeneca vaccine, conducted in roughly 2,000 people, found an under-25% efficacy against mild and moderate illness from B.1.351, failing to meet the threshold for approval. As a result, South Africa stopped offering the AstraZeneca vaccine to its citizens, instead of pivoting to Johnson & Johnson.
For now, the best way to avoid the variants is to continue following disease prevention guidelines and to get an authorized vaccine as soon as you’re eligible.
Why was the AstraZeneca vaccine temporarily paused in Europe?
First approved in the United Kingdom on December 30, AstraZeneca’s COVID-19 vaccine received World Health Organization (WHO) approval last month and has since expanded to much of Europe, plus Asia and Africa. But sporadic reports of blood clotting hindered the vaccine’s rollout, which was temporarily paused by 13 European nations and Thailand starting March 12.
On March 19, German researchers from the University of Greifswald released a statement positing that the AstraZeneca vaccine causes overactivation of blood platelets in rare cases, potentially leading to clotting.
However, the European Medicines Agency (EMA) insists that the vaccine is safe, citing under 30 reported cases of serious blood clots among 20 million people given the AstraZeneca vaccine across Europe. In its review, the agency noted that it is unclear if the events are related, and more research is needed to confirm direct causation. Both the EMA and the WHO recommend continuing with inoculations while the concerns are investigated as a precautionary measure.
Since the EMA’s report, Thailand and about half of the 13 European countries, including Germany, Italy, and France, have resumed administering the AstraZeneca COVID-19 vaccine to their citizens. (Thai Prime Minister Prayuth Chan-Ocha even became the first person to receive the AstraZeneca jab in Thailand.)
No increased likelihood of blood clotting was found among participants in AstraZeneca’s United States trial.
Which COVID-19 vaccine is the best?
“The best vaccine is the one that’s offered the day you go in for your appointment,” says Andrew Thomas, M.D., a chief clinical officer at The Ohio State University Wexner Medical Center. “All of the [available] vaccines are incredibly effective. They’re all safe.”
Each vaccine that has received FDA authorization has been proven to decrease severe illness, hospitalization, and death, the worst outcomes of COVID-19. Although the AstraZeneca vaccine hasn’t yet been approved in the U.S., research suggests it has those same effects in the places where it’s being distributed.
“The best vaccine is the one that’s offered the day you go in for your appointment.”
But remember: Vaccines aren’t a silver bullet. They must be combined with masks, hand-washing, and social distancing to work as effectively as possible, per the CDC. No matter which COVID-19 vaccine becomes available to you first, you can feel confident in its ability to protect you, as long as you continue being cautious until positive cases, hospitalizations, and deaths are significantly reduced nationwide.
In the meantime, it’s likely “that all the manufacturers are working on making their vaccines more stable at easier-to-manage temperatures,” Dr Talaat explains. Similarly, manufacturers are working toward greater protection against new variants. As their formulations change, their pros and cons will, too.
For now, we can be thankful that AstraZeneca’s vaccine is being studied more. “The next generation of vaccines, like AstraZeneca’s, which is kept at refrigerator temperatures, is a major advancement,” Dr Talaat says. “When you’re talking about distribution to the entire world, it’s much easier to do because we already keep vaccines cold. It’s a lot harder to keep things frozen.”