Comparing vaccine efficacy is like comparing apples to oranges

MORE than 115 million Covid-19 cases, resulting in about 2.6 million deaths worldwide. In contrast, more than 250 million vaccines have been administered with zero deaths directly caused by the inoculation.

This should provide an indication of the relative merit of vaccination when balanced against the risk of not vaccinating.

Several vaccines have been developed and authorised for use in more than 100 countries, with different efficacy numbers — 70, 79 and as high as 95 per cent for Oxford-AstraZeneca, Sinopharm and Pfizer-BioNTech, respectively.

However, many are confused, or unaware, about what the vaccine efficacy numbers mean.

Simply put, a 95 per cent efficacy does not mean that there is a five per cent chance of getting Covid-19. This is not true as the chance is significantly lower than five per cent.

Vaccine efficacy is determined in a randomised, controlled clinical trial so that the effects of a medical intervention can be observed with a greater level of confidence that there is minimal influence from chance or other biases.

Participants are split into two groups, with one receiving the vaccine that is being tested, and the other group getting the placebo, or dummy injection without the vaccine.

The participants are observed over a period of time, during which some would get infected, especially if the virus is widespread in the community.

In the Pfizer-BioNTech trial, 162 of the 22,000 participants in the placebo group started developing symptoms of Covid-19.

This translates into a risk of 0.74 per cent (that is, 162/22,000).

On the other hand, only eight out of the 22,000 participants who had received the vaccine started falling sick, a rate of about 0.04 per cent (that is, 8/22,000).

The vaccine had reduced the risk of Covid-19 from 0.74 per cent in the unvaccinated group to 0.04 per cent in the vaccinated group, a reduction of 95 per cent, which means the vaccine has an efficacy of 95 per cent.

Hence 95 per cent efficacy doesn’t mean that out of 100 people vaccinated, five will still get infected.

This is a common misconception.

Instead, a 95 per cent efficacy means that the vaccine is able to reduce the risk of the disease by 95 per cent.

In other words, assuming a similar disease rate in a community of one million people, the number of Covid-19-related illness would be reduced by 95 per cent, from 7,400 to 400 people.

In some instances, the vaccine trial would also report its efficacy against severe cases of Covid-19 infections.

This measures the ability of the vaccine to reduce the risk of hospitalisation and death. It is an important measure, but not to be confused with the earlier efficacy number that is more generally used and reported.

It is futile to compare vaccines on the basis of efficacy as this ignores vital details of the clinical trials. It is important to understand that different locations may have different variants.

Furthermore, there can be important differences in the demographics of the trial participants, thus comparing the efficacy of vaccines is like comparing apples to oranges.

Also, the performance of the vaccines will continue to be monitored for their safety and effectiveness throughout their use.

If vaccine efficacy measures the performance of the vaccine in clinical trials, vaccine effectiveness measures how much the vaccine reduces the disease in the real world.

As the vaccines have started to be administered worldwide in great numbers, scientists have started to gather real-world evidence confirming that the vaccines are effective in reducing Covid-19 infections and the risk of severe cases.

The other thing to remember is that being vaccinated doesn’t mean that you will never get infected. In fact, not many vaccines today provide complete protection against symptomatic disease and infection, with the rare few exceptions including the measles vaccine.

However, post-vaccination, should you still get infected, it is likely to be less severe.

Severe disease due to Covid-19 is what caused the world to come to a standstill in the first place.

Thus, significantly reducing its capability to make people gravely ill is the most important benefit of all. That itself makes it a worthy undertaking.

The writer is a Malaysian scientist living abroad who aims to create a bridge between science and society to promote evidence-based thinking