With India’s Covid-19 vaccination campaign proceeding, two terms that sound similar are prone to misinterpretation: vaccine efficacy and vaccine effectiveness. The dictionary meanings of these terms are different from the way they are used in scientific discourse. Understanding them could help readers make informed decisions when they are getting vaccinated.
Vaccine efficacy
Many people interpret vaccine efficacy to mean the percentage of people who would not get the disease after vaccination. Sometimes, it is also interpreted as the percent of possibility that a vaccinated person would be protected from the disease. However, it is not as straightforward.
To understand vaccine efficacy, one has to first know a little bit about randomised controlled trials, a type of research design, commonly used for clinical trials – in this case, for Covid-19 vaccines. In RCTs, the study participants are randomly assigned into two comparable groups. In one, the experimental group, the actual vaccine is administered. The other, the control group, gets a placebo – a neutral, unharmful substance that is not a vaccine.
The process takes place in optimal conditions, which means that it occurs in an environment that is scientifically controlled and is similar for both experimental and control groups. If the outcome in the experimental group, in statistical terms, is significantly better than the outcome in the control group, the vaccine would be called efficacious.
Interpreting vaccine efficacy
The efficacy of Covid-19 vaccines is mentioned in percentages. The way it is calculated and interpreted is a little more complicated.
If, for instance, a vaccine is 90% efficacious, this means that out of the total number of people enrolled in the vaccine trial, if about 1% of the people in the control group (who received the placebo instead of the vaccine) get symptomatic Covid-19, only 0.1% of those in the experimental group (who received the vaccine) would get symptomatic Covid-19.
Efficacy is a comparative measure of vaccine performance and not its absolute measure. Ninety per cent efficacy simply means that if 10 people out of 100 unvaccinated people normally get symptomatic disease, only one out of 100 vaccinated ones would get it. The formula used to measure vaccine efficacy is 100 x [1 - (attack rate in the experimental group divided by attack rate in the control group)]. In our example, it would be 100 x [1 - (1/10)] = 90%.
India is currently using two vaccines for Covid-19: Covishield, produced by the Serum Institute of India, and Covaxin, produced by Bharat Biotech. Based on the first clinical trial results, Covishield was 90% efficacious in preventing symptomatic Covid-19 after two doses as per the prescribed schedule, while Covaxin is 81% efficacious.
However, the Covishield vaccine efficacy has been lowered over time and the latest results show that it is 76% efficacious against symptomatic Covid-19. Efficacy rates change based on the gap between the first and the second dose of the vaccine. India has changed the schedule of Covishield vaccine – the recommended gap between the two doses is now six-eight weeks, up from four weeks earlier.
The Sputnik V vaccine, being manufactured in India by Dr Reddy’s Laboratories, is due to be used in the country soon. Its efficacy is said to be 91.6%, next only to the vaccines by Pfizer-BioNTech and Moderna, the two vaccines that are not currently used in India.
Efficacy in what terms?
Vaccines are primarily aimed at preventing a disease due to infection and in turn, its spread. They stimulate an immune response within the body without causing a real infection, which in turn protects the body should there be a real infection later. Therefore, efficaciousness of a vaccine is often measured in terms of the level of immune response generated in the body. For instance, the Oral Polio Vaccine stimulates the immune system to produce anti-poliovirus antibodies against three types of poliovirus. These antibodies will protect the body should there be an infection of poliovirus at a later stage.
In case of Covid-19 vaccines, the efficacy is measured differently. It has been little over a year that we have known about the novel coronavirus, its different strains and the disease. The immune response that the virus stimulates in human body could either be antibody-mediated or cell-mediated. Which kind of immune response is activated when and why is still being studied. Therefore, it has been not easy to measure the efficacy of Covid-19 vaccines in terms of the immune response produced.
However, a vaccine was urgently needed to contain the disease, if not to completely prevent it. Consequently, vaccine performance was measured in terms of the symptoms averted. This way, at least severe symptoms and deaths due to Covid-19 can be potentially prevented. This is the reason we hear that the vaccine does not guarantee complete protection from Covid-19 after vaccination and that it only prevents symptomatic disease.
Limited knowledge about the behaviour of the virus and human immune response, and athe very short time in which the vaccines have been developed and tested are some of the key reasons these vaccines do not assure complete prevention of the disease. This is why these vaccines have got “emergency use approvals” in most countries and vaccination is made voluntary.
Vaccine effectiveness
Vaccine efficacy and effectiveness, both conceptually measures of a vaccine’s performance, are different in terms of the conditions in which they are measured. But these conditions may well be a deciding factor for using a vaccine in the long run.
The optimal conditions in efficacy studies are used for recruiting the participants for a vaccine trial in order for the experimental and control groups to be comparable. Depending on the aim of the trial, specific eligibility criteria such as age, setting, prior exposure could be used for creating optimal conditions during efficacy trials. These are akin to the laboratory research, which takes place in controlled or ideal conditions.
Vaccine effectiveness, however, is measured in real-world settings. Vaccines are administered at various places (primary care centres, vaccination camps and more), to people of various age groups, sex, varied levels of prior exposure, comorbidities and so on. These uncontrolled conditions give a broader, more detailed picture of vaccine performance in a range of conditions. Considering this broad array of biological, social, genetic and environmental factors, vaccine effectiveness is aimed at measuring several aspects of vaccine outcomes.
These include (but are not limited to) differences in clinical outcomes based on personal, demographic and clinical parameters, adverse event following immunisation, cost effectiveness (disease cost vs vaccine cost), side effects and more. In other words, effectiveness is a measure of the real-world application of a laboratory experiment. In addition, because of the large sample size in effectiveness studies, it becomes a well-rounded measure of vaccine performance.
Vaccine effectiveness studies are critical in making decisions about the long-term use of vaccines.
In the case of Covid-19 vaccines, owing to the limited time period for which they have been used, effectiveness studies are rare. Much data on effectiveness of vaccines used in India is not yet available. As a result, the extent to which these vaccines are effective in the real-world settings and in the long run.
Why it matters
In a world of quick communication, readers are bombarded with information but are not always equipped to verify its authenticity. This is especially true in matters of health, which are frequently discussed but in a way that could lead to misinterpretation. These frequent misinterpretations may lead to mistrust, doubt, politicisation, vaccine hesitancy and fear. This could ultimately impact personal and public health outcomes.
Note: Vaccine efficacy percentages are based on the currently available research. Some media reports may mention different efficacy percentages.
Dr Sumedh MK is a public health professional, currently working as a consultant with the Maharashtra government’s public health department.