How the Sinovac vaccine works

Chinese private company Sinovac has developed a coronavirus vaccine called CoronaVac. On December 23, Brazilian researchers announced that the vaccine was more than 50 percent effective. Full details of Sinovac’s phase 3 tests are expected in early January.

A vaccine made from coronavirus

CoronaVac works by teaching the immune system to make antibodies against the SARS-CoV-2 coronavirus. Antibodies bind to viral proteins, such as so-called peak proteins, which are on their surface.

To create CoronaVac, Sinovac researchers began to obtain samples of the coronavirus from patients in China, Britain, Italy, Spain and Switzerland. A sample from China ended up serving as the basis for the vaccine.

Killing the Virus

The researchers cultivated large stocks of coronavirus in monkey kidney cells. Then they drenched the virus with a chemical called beta-propiolactone. The compound deactivated coronaviruses by binding to its genes. Inactivated coronaviruses could no longer replicate. But its proteins, including the peak, remained intact.

The researchers then removed the inactivated viruses and mixed them with a small amount of an aluminum-based compound called an adjuvant. Adjuvants stimulate the immune system to increase its response to a vaccine.

Inactivated viruses have been used for more than a century. Jonas Salk used them to create his polio vaccine in the 1950s, and they are the basis for vaccines against other diseases, including rabies and hepatitis A.

Requesting an immune response

Since the coronaviruses in CoronaVac are dead, they can be injected into the arm without causing Covid-19. Once inside the body, some of the inactivated viruses are swallowed up by a type of immune cell called an antigen-presenting cell.

The antigen-presenting cell separates the coronavirus and displays some of its fragments on its surface. A so-called auxiliary T cell can detect the fragment. If the fragment fits into one of its surface proteins, the T cell becomes activated and can help recruit other immune cells to respond to the vaccine.

Making Antibodies

Another type of immune cell, called a B cell, can also find the inactivated coronavirus. B cells have surface proteins in a wide variety of forms, and some may have the right shape to attach to the coronavirus. When a B cell is fixed, it can pull part or all of the virus inward and have fragments of coronavirus on its surface.

An auxiliary T cell activated against the coronavirus can cling to the same fragment. When this happens, cell B is also activated. It proliferates and releases antibodies that have the same shape as its surface proteins.

Stopping the virus

Once vaccinated with CoronaVac, the immune system can respond to infection with live coronaviruses. B cells produce antibodies that stick to invaders. Antibodies that target the spike protein can prevent the virus from entering cells. Other types of antibodies can block the virus by other means.

Remembering the virus

Although CoronaVac can offer some protection against Covid-19, no one can say how long that protection lasts. It is possible that the level of antibodies will decrease over the months. But the immune system also contains special cells called memory B cells, which can retain information about the coronavirus for years or even decades.

Vaccine Timeline

January 2020 Sinovac starts to develop an inactivated vaccine against coronavirus.

June Phase 1/2 trials in 743 volunteers found no serious adverse effects.

July Sinovac launches a Phase 3 test in Brazil, followed by others in Indonesia and Turkey. Reuters reports that the Chinese government has given Sinovac vaccine emergency approval for limited use.

October Officials in the eastern Chinese city of Jiaxing have announced that they are giving CoronaVac to people in relatively high-risk jobs, including medical workers, port inspectors and public service employees.

October 19 Authorities in Brazil say Sinovac is the safest of the five vaccines they are testing in Phase 3 of the tests.

November Sinovac publishes the details of his phase 1/2 trial in a medical journal, showing relatively modest antibody production. Only a Phase 3 test will demonstrate whether this is sufficient to protect people at Covid-19.

November 19 The Brazilian government announces that it stopped the Sinovac trial in the previous month due to an adverse event. The details of the break were unclear, raising suspicions that the policy was involved. Two days after the announcement, the trial was resumed. The Brazilian trial recorded enough cases of Covid-19 to allow researchers to determine the effectiveness of Sinovac. They hope to release their results by December 23.

December Sinovac says it expects to manufacture 300 million doses in 2020 and increase capacity to an annual production of 600 million doses.

December 23 Brazilian researchers announce that CoronaVac is more than 50 percent effective.

Sources: National Center for Biotechnology Information; Science; The Lancet; Lynda Coughlan, University of Maryland School of Medicine; Jenna Guthmiller, University of Chicago.

Tracking Coronavirus