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The number of vaccines being used to fight the COVID-19 pandemic continues to grow, with several international candidates that could soon enter the U.S. market.

The University of Oxford and AstraZeneca have teamed up in the United Kingdom to produce a recombinant vaccine made from a modified chimpanzee adenovirus. Another candidate is from China’s Sinovac Biotech. The Sinovac vaccine uses more traditional vaccine technology, relying on inactivated virus particles to produce an immune response.

Neither of these vaccines has been approved for regular or emergency use in the United States yet. Numerous trials are underway and applications could be presented to the Food & Drug Administration (FDA) in the future to add to the nation’s arsenal against COVID-19. Both vaccines are also being reviewed by the World Health Organization.

AstraZenecaSinovac
Official nameAZD1222CoronaVac
Effectiveness76%56%–65%
Dosage2 doses, 28 days apart2 doses, 21 days apart
Storagestandard refrigerationstandard refrigeration
Typechimpanzee adenovirusinactivated virus
Approval statusU.S. and international clinical trialsVarious international clinical trials
U.S. availability300 million doses owned, but not approved for usenone

Both the AstraZeneca and Sinovac vaccines are still undergoing clinical trials across the globe, so there is some uncertainty as to how effective each vaccine is against COVID-19.

There was some controversy over the effectiveness of AstraZeneca’s vaccine in late 2020 when it was revealed that some people in the early study groups only received half doses of the vaccine.

While AstraZeneca claimed that the vaccine was 70-percent effective, it was later disclosed that the effectiveness was 62 percent in people who received two full doses, and closer to 90 percent in people who received one half and one full dose. AstraZeneca used these two percentages to average an effectiveness rate of 76 percent.

Data on Sinovac’s CoronaVac vaccine is limited since a number of international studies on the vaccine are still underway. In one report, investigators report that 97 to 100 percent of people who received the vaccine in clinical trials developed antibodies to COVID-19, but not all of the immune response markers measured in other studies were investigated for the CoronaVac vaccine.

Another report on the effectiveness of CoronaVac reveals that phase 3 clinical trial results have yet to be released by Sinovac, but a trial in Chile shows an effectiveness rate of just 56.5 percent after full vaccination with CoronaVac.

Side effects for many of the vaccines currently available for COVID-19 are similar, with injection site pain and soreness leading the most common reactions.

For the CoronaVac vaccine, injection site pain and soreness was the most common side effect, reported by 17 to 21 percent of people who received various doses of the vaccines.

In most cases, the reactions were mild and resolved within 2 days. There was one case in the phase 1 trial for the vaccine where a recipient developed an allergic skin reaction with welts but was treated with antihistamines and steroids and resolved in 3 days.

Systemic reactions that affected more than just the injection site included:

  • fatigue
  • diarrhea
  • muscle weakness

These symptoms were reported far less than injection site pain, according to trial data.

The AstraZeneca vaccine attracted a lot more attention for its side effects after some people developed blood clots after vaccination.

Administration of the vaccine was paused in many parts of the world as regulators investigated the clots, concluding they were a very rare side effect — impacting about 86 people out of the 25 million who had received the vaccine. European regulators allowed AstraZeneca to resume its vaccination program, but some countries restricted its use to older adults.

Other side effects of the AstraZeneca vaccine, which were also rare, included cases of:

  • inflammation around the spinal cord
  • hemolytic anemia
  • high fevers

All of these symptoms resolved without additional problems. More common side effects included:

  • injection site pain
  • mild tenderness
  • fatigue
  • headache
  • muscle ache
  • chills
  • fever

Most of these reactions were mild, according to trial data, and resolved within a day or so after vaccination.

Vaccination in general works by using small bits of information about a disease — like a protein spike or inactivated virus particle — to teach your immune system how to recognize and destroy the active virus.

Unlike the Pfizer and Moderna vaccines that use mRNA technology (pieces of COVID’s genetic code) to create immunity, the AstraZeneca and Sinovac vaccines use a more traditional route. This means that actual virus particles or genetic materials are combined with other materials to introduce small, harmless pieces of the virus into your body. Your immune system is able to use this information to design a defense, making it better equipped to fight the live virus.

Similar to the Johnson & Johnson COVID-19 vaccine, the AstraZeneca vaccine relies on a chimpanzee adenovirus to carry spike proteins from the coronavirus into your body to create an immune response.

The Sinovac CoronaVac vaccine also uses a virus to create immunity. However, instead of adenovirus, the vaccine uses inactivated particles of the SARS-CoV-2 virus that causes COVID-19.

What is a chimpanzee adenovirus?

Adenovirus is a very common virus. Usually causing mild illnesses like the common cold, adenoviruses are widespread and efficient. There are more than 50 types of adenoviruses that cause respiratory infections, and they are good at moving into and attacking the body in various ways. A chimpanzee adenovirus is an adenovirus that causes these infections in chimpanzees.

When modified for use in vaccines, these viruses are very efficient at helping to produce immune responses. These types of vaccines, like the AstraZeneca vaccine, are known as viral vector vaccines.

What does inactivated mean?

When a vaccine is made from an inactivated virus, this means that the part of the virus that causes disease is destroyed, but its basic genetic information remains.

When injected as a vaccine, the inactivated virus will train your immune system to fight the disease it causes, but won’t make you sick. Influenza, polio, and rabies vaccines use inactivated viruses to create immunity, but don’t create immune responses as strong as other types of vaccines.

Researchers are throwing everything they’ve got at the COVID-19 pandemic, and there are a number of different companies making many variations of a vaccine to fight the virus.

While there are only three vaccines currently approved for use in the United States, vaccine developers around the world are working on solutions, too. China’s Sinovac CoronaVac and the United Kingdom’s Oxford-AstraZeneca vaccines are two examples of these international options being studied across the globe.