Quality inspectors on Sinovac vaccine production lines use digital technology platforms to monitor the quality and efficiency of vaccine packaging on Sunday. Photos: Li Hao/GT

Sinovac's COVID-19 vaccine is 100 percent effective in preventing severe and moderate infections, 77.96 percent effective in preventing mild cases, and has an overall efficacy of 50.4 percent in Brazil's final-stage trials.

Experts say the result is good enough considering almost all participants in Brazil are high-risk medical workers, and the 77.96 efficacy for mild-case protection means the vaccine can reduce 78 percent of people from needing hospitalization.

We have today one of the best vaccines in the world, Dimas Covas, director of the Butantan Institute in Brazil, said during a news conference on Tuesday.

Almost all of participants in the trials are medical workers at high risk of contracting the coronavirus. "The vaccine was able to 100 percent prevent severe illness in such a high-risk population, and effectively control the morbidity, and protect at least 50% of people from infection, which is good enough," Wang Guiqiang, director of the Infectious Disease Department at Peking University First Hospital, told the Global Times on Tuesday.

"Currently, the manufacturer requires a 0-14 vaccination doses program (Day 1 for first dose, Day 14 for second dose), but this probably cannot bring the highest level of antibody which then partly lowered the efficacy result. That's why some manufactures ask for a 0-21 vaccination program. But overall, its protective effects in such high-risk population is good enough," Wang stressed.

The manufacturer of the vaccine, named CoronaVac, requires recipients receive two doses 14 days apart, and if this was not adhered to, the efficacy could have been higher, Wang said.

The most frequent systemic effects were headache and myalgia and those events were mild and did not interfere with the participants' daily routine. There were no severe events associated with the vaccine.

Adverse events were very rare, Dimas Covas, director of the Butantan Institute, which cooperates with Sinovac on the trials in Brazil, said during a news conference on Tuesday.

Covas said it predicts that higher effectiveness will be seen in community use, and that it expects the vaccine will have a very high effectiveness among the general population similar to trials in other countries.

We chose the broadest number of symptoms. We would like to capture even the most simple symptoms. Those symptoms have not been included in similar studies, he said.

A 77.96 efficacy rate for mild-case protection means the vaccine can reduce 78 percent of people from needing hospitalization, effectively avoiding medical system collapse, Wang said.

Sinovac's vaccine has been undergoing Phase III clinical trials in Brazil, Indonesia and Turkey, which have designed different clinical plans based on their own domestic conditions, reads a document the Global Times obtained from Sinovac.  

The plans, using vaccines from the same batch and the same immunization schedules, achieved different results of efficacy: 91.3 percent in Turkey; 100 percent protection against severe symptoms and 78 percent for mild cases in Brazil; and 65.3 percent in Turkey, which is normal and shows objectivity of clinical trials, according to the document.  

Despite the difference in efficacy, the general conclusion is that the vaccine is protective, especially in preventing people from getting medium and severe symptoms, noted the Sinovac document. 

For example, seven people who were hospitalized or critical cases in Brazil were all in the placebo group, which means the vaccine's protection rate for severe cases is 100 percent. 

Meanwhile, in Turkey, the six hospitalized cases were also all in the placebo group, also showing a 100 percent protection rate for these cases. 

These results demonstrate that the Sinovac vaccine would not lead to antibody-dependent enhancement which could bring about the failure of vaccines and infections that are even more serious.

Volunteers for Sinovac's vaccine in Brazil are all medical workers who are at the highest risk of infection. It is also the world's first Phase III clinical trials conducted among only medical workers, reads the Sinovac document.

The trials in Brazil also recorded the biggest ratio of slight symptom cases - 85 percent - as the team used highly sensitive Polymerase Chain Reaction (PCR) test.

There were also 1,394 volunteers who received two doses three weeks apart instead of two weeks apart. Efficacy among the former group is about 20 percent higher than the latter group, which means the overall efficacy could improve if all volunteers follow a 21 day schedule for second doses, Shao Yiming, one of the vaccine developers and AIDS expert at the China's CDC, told the Global Times on Tuesday.

Medical workers are in a high exposure population, which would decrease vaccine's protection efficacy, Shao noted, citing an example of previous international Phase III clinical trials on an AIDS vaccine.

The trials show that the vaccine's efficacy is 30 percent in Thailand but 0 percent in South Africa while AIDS incidence rate in South Africa is four times that of Thailand. 

The more a vaccine is attacked by the virus, the lower its efficacy would be, which could explain the comparatively low efficacy of COVID-19 vaccines in medical workers, Shao explained. 

Recently, AstraZeneca released detailed data on its vaccine which shows it has a 70 percent efficacy for people showing symptoms and 27 percent efficacy for silent virus carriers. 

Meanwhile, the incidence rates in placebo groups of Pfizer and Moderna's Phase III clinical trials were 7.29 percent and 3.34 percent respectively, which were lower than the US' overall incidence rate during the same period. The phenomenon is abnormal under a high-sensitivity supervision system in clinical trials, according to Shao. 

It is notable that the infection risk of Sinovac's trial environment is three times higher than Pfizer's and six times higher than Moderna's, Shao stressed. He said that the protection rate of a vaccine not only depends on its efficacy but also its environment of infection risk. He thought that Sinovac's vaccine would show better protection rate in the public.  

The exceedingly high-sensitive system Sinovac used would drag down efficacy, Shao said. 

CoronaVac's clinical trials on children and pregnant women will be conducted soon, according to Brazilian official.