The efficacy rates for various Covid-19 vaccines cannot be usefully compared as each was tested and studied under different circumstances and different times, Thailand’s leading virologist said Thursday, encouraging people not to hold off and to get whatever vaccine is available to them.
“The effectiveness of a vaccine depends on a variety of factors,” Dr Yong Poovorawan, the chief of the Centre of Excellence in Clinical Virology at Chulalongkorn University and one of the country’s top virologists leading the Covid-19 situation, shared on Facebook.
Dr Yong’s latest explanation comes as the government is scrambling to expand its Covid-19 vaccine programme amid the country’s worst outbreak yet.
Over the past two weeks doctors and local media have highlighted how getting vaccinated with any of the jabs available is much safer than not getting vaccinated and risking infection. Many citizens, however, are still reluctant.
Dr Yong explained why the numbers used to asses the performance of the different vaccines do not allow for a meaningful comparison between them.
The prevalence of the disease at the time of the study should be taken into account, he wrote. If the prevalence is high, then the performance figure will be lower. For example Pfizer, at 95 per cent effectiveness, and Johnson & Johnson, at 66.3 per cent effectiveness, although both from the US, are impossible to compare as the tests for Pfizer were conducted before those for Johnson & Johnson, when the outbreak was at its peak.
Second is the selection of the population in the study. If a high-risk population is used, then the vaccine efficacy rate is lower. For example, studies for the Sinovac vaccine conducted in Brazil on high-risk groups such as front-line health care workers showed a lower efficacy than the same study conducted in Turkey, which used a low-risk general population.
Third is taking the severity of the disease into account. If the severity of the disease on a population is high, then the efficacy rate will be high. Dr Yong explained that, for example, the Sinovac vaccine was tested in Brazil when the symptoms and severity of the virus were low and considered grade 2 (not requiring medical attention) on the World Health Organization (WHO) scale, leading to a lower efficacy rate. This is incomparable to the other tests conducted on other vaccines, where most cases were considered grade 3 (need medical attention) and higher.
Vaccines do not always completely prevent infection, but they train the body’s immune system to fight it off more quickly, and reduce the severity of symptoms in those who do develop any.
Lastly, the strains of each virus variant must also factor in. Studies for Pfizer and Moderna, for example, were conducted before Johnson & Johnson’s and before the virus mutated any further, which have contributed to its higher efficacy rate. Dr Yong gave an example of how the efficacy rate of Pfizer was significantly lower (20 per cent) against the South African (501.V2) variant. The same goes for a recent case in India where a patient inoculated with two doses of the Pfizer vaccine contracted the Indian Covid-19 variant and died.
“Practically, in order to look at each of the vaccines, we must study all the data and information available,” concluded Dr Yong. “But sometimes we do not have all the information.”
“Therefore, we would advise against using these numbers, that were conducted differently, at different time periods, to compare which vaccine is better than the other.”
The professor and virologist stressed the need in Thailand for all citizens to get inoculated.
Dr Yong is the latest doctor to come out and clarify why the vaccines are not comparable, demystifying the surrounding myths and emphasizing the need for the public to better understand the situation and get vaccinated.
His post came as millions in other countries are also comparing the effectiveness of the various vaccines. US media company Vox released a similar explanation in March.
Listen to this story