Corona Daily 190: On Efficacy and Herd Immunity

In my research, I increasingly come across the words “efficacy” and the more awkward “efficacious”. When newspapers say the Pfizer-BioNTech vaccine has a 95 percent efficacy, why don’t they say it is 95% effective? Because in the pharma and medical world, the two terms are technically different. 

Efficacy is the number we get from a clinical trial, in research settings. Though the trials had tens of thousands of volunteers who lived lives as they normally do, it wasn’t the “real world” test of the vaccine.

Effectiveness is what happens in the real world.

In many other fields of life, we know there may be a difference between the controlled setting and the real world. In cricket, batting in the nets, and playing in a real match in front of crowds. Between rehearsals and on-stage theatrical performances. Projections in the corporate board rooms, and actual company performance.

As we already know, some pregnant women are opting to take the vaccines. They were not included in any trials. Also, the racial and ethnic composition of the trials didn’t exactly represent the populations where trials were conducted.

As a rule, the effectiveness of the vaccines will be lower than the efficacy. The coming years would show how effective Moderna, Pfizer or Oxford were. Neither the efficacy nor the effectiveness can accurately tell you your personal risk. It depends on your immune system, health conditions, and exposure to the virus. At this stage in the pandemic, experts are urging people not to focus on the efficacy numbers, but just go ahead and take whichever authorized vaccine that is offered.

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Herd immunity is another popular expression few people had heard before the pandemic. When a significant percentage of the population is vaccinated, or has recovered naturally, it is difficult for the virus to spread. Think of an unsuspecting village, in which a burglar goes from house to house robbing households. In the beginning, he can enter any house of his choice and take cash and jewelry. Over time, he can’t enter houses which are well locked (vaccines) and houses he has already broken into (antibodies). Finally, unable to penetrate an unburgled house, he leaves the village or dies. I imagine the virus to be like that burglar.

Depending on the virus, there is a certain % that needs to achieve immunity before herd immunity is reached. That % is called the “herd immunity threshold”. Measles, a particularly contagious disease, slows down only after 95% of people become immune. Herd immunity doesn’t protect against all vaccine-preventable diseases. Tetanus, for example, is caught from bacteria in the environment, rather than from other people. No matter what percentage of population is vaccinated for tetanus, it doesn’t protect an unvaccinated person; for tetanus herd immunity is not possible.

Herd immunity is important for people with low or compromised immunity, people with HIV, newborn babies too young for a vaccine, elderly or very ill people who can’t safely get vaccinated.

Currently, India is doing very well, with low numbers of cases and deaths, and except for mask-wearing, life is returning to normal. In Delhi, a recent survey suggested more than half of Delhi’s population had antibodies. Many Indians attribute India’s current well-being to the presumed herd immunity. I would like to wait till the monsoon season is over to pass that judgement. June to September is India’s annual infection season, just like winter is Europe’s and America’s. In my view, if India’s numbers remain low till the end of September, it may be out of the woods.

Ravi