The lower protection rates of China’s coronavirus disease 2019 (COVID-19) vaccines have raised concerns about shots that are key to inoculation rollouts from Brazil to Indonesia, especially after their efficacy was questioned by one of the most senior Chinese health officials.

Bloomberg spoke to two experts about the vaccines, zeroing in on the Sinovac Biotech Ltd. shot, which has been the focus of a crisis in confidence after it posted efficacy levels of just above 50% in a final-stage trial in Brazil — the minimum required by leading global drug regulators. Other Chinese immunizations have delivered rates between 66% to 79%, still far below the shots developed by Pfizer Inc., Moderna Inc., and even Russia’s Sputnik vaccine that have logged protection rates of more than 90%.

More than 30 countries have rolled out the Sinovac shot, most of them in the developing world, but also Hong Kong. The vaccine is key to China’s own mammoth push to inoculate 560 million people — 40% of its population — by the end of June.

The good news is the vaccines work extremely well in combating severe COVID-19 infections, according to Fiona Russell from the Murdoch Children’s Research Institute in Melbourne and Paul Griffin, a professor from the University of Queensland in Brisbane. Bloomberg asked them key questions about the merits of the Sinovac shot. Their comments have been edited and condensed for brevity.

HOW EFFECTIVE IS THE SINOVAC VACCINE REALLY?
Russell: The Sinovac study was to look at how the vaccine works against the entire range of clinical symptoms, from mild infections to severe ones, including death. The efficacy data of about 50% is for very mild disease, requiring no treatment. For infections requiring some medical intervention, it’s about 84% and for moderate-to-severe COVID cases, it’s 100%.

That’s what you expect from COVID vaccines — higher efficacy against more severe infections and lower against milder ones. From what I can see, it looks like a very worthwhile vaccine. The problem with the data is it didn’t include many old people or many people with co-morbidities.

Griffin: My impression is the efficacy seems likely to be above the minimum threshold set by the World Health Organization but less efficacious than a number of other leading vaccine candidates. While it may be less effective against symptomatic COVID (mild infections), the efficacy in severe cases is very high, which is an incredibly valuable property of any vaccine, including this one.

WHY DOES IT HAVE VARYING EFFICACY RATES IN DIFFERENT STUDIES?
Russell: This has caused a bit of confusion. The trials were done in Brazil, Indonesia, and Turkey. Each of the studies have come with different results. In Brazil, you’ve got the P.1 variant circulating and so potentially the vaccine efficacy may be different due to that. I’m not familiar with what was circulating in Turkey or in Indonesia at the time of the study but that could obviously change the results.

Also, it’s very hard to directly compare results from different trials because they have to be interpreted in the context of the study design. The case definitions — the way a COVID-19 case is identified in a clinical trial — they used were different, so were the endpoints of the studies.

Griffin: This shows how variables in clinical trials can impact the results. Different strains circulating in a nation are a big variable that can alter the efficacy readout. Virus strains are vitally important in determining vaccine efficacy.

WHAT CAN BE DONE TO IMPROVE THE VACCINE?
Russell: The recommended schedule for two doses of the Sinovac shot in the trial was two weeks, though some people spread it longer than that, even one month apart. Theoretically, tweaking the schedule is definitely an option. Mixing vaccines — starting with one type and then boosting it with a different one — is another option but there’s a lot more we need to know about those variations.

Griffin: It’s very likely that the dosing interval is critically important and certainly sounds like it was a variable in Sinovac’s clinical trials as well. We need another clinical trial to ascertain whether adding another booster increases efficacy. In terms of heterologous boosting — using a mix of different vaccines — the research is underway. A lot of people are optimistic but until we have the data, it’s hard to know.

IS IT POSSIBLE TO REACH HERD IMMUNITY WITH SINOVAC?
Russell: That depends on a number of things — the circulating variant, the coverage required and the degree to which the vaccine prevents virus transmission. But we don’t have a lot of data yet on transmission.

Griffin: If we have a less effective vaccine, we are going to need to inoculate more people to achieve that. Its potential to reduce virus transmission is likely to be less too but again, without good data it’s hard to be sure. With very high coverage, even a moderately effective vaccine is perhaps better than a highly effective vaccine with poor coverage. So, it depends on the uptake among other factors.

SHOULD PEOPLE TAKE THE SINOVAC VACCINE?
Russell: I would have no hesitation, provided it secures regulatory authority approval. For protecting people from ending up in hospitals, Sinovac’s shot looks terrific as the efficacy data for preventing hospitalization and death is 100%.

Griffin: Yes. While it may mean that we don’t reduce the cases overall or achieve herd immunity, we will still be successful in reducing severe COVID infections and therefore, the burden on the health care system. If the vaccine is approved by the relevant regulators, then I think we need to have faith that it’s safe and effective in that population.

If there are multiple vaccines available, then people could potentially opt for one that has demonstrated superiority in clinical trials. But if that’s not possible, then having this vaccine is better than not being vaccinated at all. — Bloomberg