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expert reaction to reported results of the Sinovac COVID-19 vaccine from a trial by the Butantan Institute in Brazil

A trial by the Butantan Institute in Brazil reported an efficacy of 50.38% for China’s COVID-19 Vaccine, Sinovac.


Prof Paul Hunter, Professor in Medicine, The Norwich School of Medicine, University of East Anglia, said:

“It is difficult to be too sure what the real efficacy is of the Sinovac vaccine given the limited data presented results. The Sinovac vaccine is an inactivated vaccine.

“The case definitions in the report are not absolutely clear. A case was defined as one or more of a range of symptoms some of which would not be characterised as COVID defining symptoms, though all needed a positive PCR swab (as far as I can tell).

These were

  • Grave Hospitalização / UTI  – Grave hospitalised
  • Moderados Hospitalização – Moderate but hospitalised
  • Leve Precisa de assistência – Mild but assistance needed
  • Muito leve Não precisa de ajuda – Very mild no assistance needed
  • Assintomáticos – Asymptomatic

“The estimates of efficacy varied depending on how severe the confirmed illness was. In this regard the headline efficacy figures for the three vaccines currently registered in the UK was based on any symptomatic infection so probably “Muito leve Não precisa de ajuda” and all more severe categories.

“From the information in the presentation the vaccine seemed to have no obvious concerns around adverse reactions.

“The overall efficacy directly comparable to the definitions used in the UK licenced vaccines was only 50.4% compared to about 70% for the Oxford vaccine and about 95% for the two mRNA vaccines. However, the vaccine was more effective at reducing the risk of more severe disease needing medical attention (78%) and hospitalisation (100% but on too few numbers to be confident of the exact efficacy).

“So it would appear that the Sinovac is rather less effective than the UK licenced vaccines. Having said that, six months ago we would have been delighted with a vaccine that had 50% efficacy. However, it should be noted that the two doses appear to have been given with just two weeks between doses. If so, this would be very short for any vaccine and in my view overly short and may have contributed to the less than impressive reported efficacy.

“But one more optimistic aspect is that efficacy seems to be greater for more serious disease. This finding is similar to what was found in the Oxford vaccine as well so headline efficacy figures for either vaccine may underestimate the value of both these vaccines in preventing serious disease. Let’s hope so.”


Dr Julian Tang, Honorary Associate Professor/Clinical Virologist, University of Leicester, said:

“So from the Brazilian slides on this Sinovac vaccine trial, it appears that the numbers of infected/symptomatic COVID-19 cases in the trial were stratified into mild (85 in the vaccine arm vs. 167 in the placebo arm), moderate (7 vs. 31), and severe (0 vs. 7).

“With vaccine efficacies calculated as 1 – relative risk (RR) of the vaccine (Sinovac) being able to prevent mild (50%), moderate (78%), and severe (100%) symptomatic COVID-19 cases.

“The mild, moderate, severe disease end-points used here may differ slightly from the other vaccine trials but the overall range of efficacy values for the mild/moderate/severe COVID-19 endpoints may actually not differ that much from the Oxford-AstraZeneca vaccine efficacy range of 60-90%, with their low and standard dose formulations.”


Prof Lawrence Young, Virologist and Professor of Molecular Oncology, University of Warwick, said:

“The reporting of the Sinovac trial of Coronavac (an inactivated whole virus vaccine) is very confusing. The revised 50.4% efficacy rate from the Brazilian clinical trial includes those who had ‘very mild’ cases of COVID-19 whereas original reports indicated an overall efficacy of 78% for mild to severe cases. The same vaccine has been trialled in Turkey (reported efficacy 91.25%) and Indonesia (reported efficacy 65.3%). The data from China only report a 94.9% seroconversion rate in a Phase I/II clinical trial with no report on efficacy. It is difficult to interpret all this information without seeing the full datasets. This highlights the problem of issuing data by press release rather than publication in a peer-reviewed journal. It also emphasises the different approaches to requirements for regulatory approval. It is interesting that both the Chinese and Russian vaccines are being distributed to countries such as Brazil and India in response to the perception that the ‘Western’ vaccines are being pre-purchased and hoarded by the ‘rich’ countries.”



All our previous output on this subject can be seen at this weblink:



Declared interests

None received.

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