DNA Explainer: Know the difference between efficacy and effectiveness of Covid-19 vaccine

Even as the world reels under the Covid-19 pandemic, all eyes are on the pharmaceutical companies who are in the race to make coronavirus vaccines.

However the real test lies ahead, in the fact as to how effective a vaccine is going to prove in the fight against Covid-19. 

While Pfizer and BioNTech announced their vaccine had an efficacy rate of 95 per cent, Moderna put its efficacy rate at 94.5 per cent. Russian maker for Sputnik vaccine claimed their efficacy rate over 90 per cent.

Scientists were encouraged and even surprised by the magnitude of these claims, public health officials finally saw what could be the beginning of the end of the pandemic

Food and Drug Administration said that this was a very positive sign, something that was not expected. 

'These are game changers,' said Dr. Gregory Poland, a vaccine researcher at the Mayo Clinic. 'We were all expecting 50 to 70 per cent efficacy.'

Indeed, the Food and Drug Administration had said it would consider granting emergency approval for vaccines that showed just 50 per cent efficacy.

But the same public health experts who are encouraged by the positive vaccine results are also warning that vaccines aren’t the panacea that many are desperately hoping they will be.

And that, even after more people get vaccinated, wearing masks and maintaining social distance will still remain the norm.

Exactly how the vaccines will behave depends a lot on factors such as how many people will get vaccinated or whether vaccinated people can get asymptomatic infections.

Here's what you need to know about the actual effectiveness of these vaccines as explained to us by Money Control.

What does 95 per cent effective vaccine mean?

Researchers vaccinate some people and give a placebo to others. They then wait for participants to get sick and observe how many fell ill from both the groups.

In the case of Pfizer, for example, the company recruited 43,661 volunteers and waited for 170 people to come down with symptoms of COVID-19 and then get a positive test. Out of these 170 participants, 162 had received a placebo shot, and just eight had received the real vaccine.

From these numbers, Pfizer’s researchers calculated the fraction of volunteers in each group who got sick. Both fractions were small, but the fraction of unvaccinated volunteers who got sick was much bigger than the fraction of vaccinated ones. The scientists then determined the relative difference between those two fractions.

According to scientists the difference between the people who fell sick after vaccination and those who fell sick without it, is called the efficacy.

If there's no difference between the vaccine and placebo groups, the efficacy is zero. If none of the sick people had been vaccinated, the efficacy is 100 per cent.

A 95 per cent efficacy is certainly compelling evidence that a vaccine works well. But that number does not tell you what your chances are of becoming sick if you get vaccinated. And on its own, it also doesn't say how well the vaccine will bring down COVID-19 across the World.

Difference between efficacy and effectiveness?

Efficacy and effectiveness are related to each other, but they're not the same thing. Efficacy is just a measurement made during a clinical trial. 'Effectiveness is how well the vaccine works out in the real world,' said Naor Bar-Zeev, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health.

The mismatch comes about because the people who join clinical trials are not a perfect reflection of the population at large. Out in the real world, people may have a host of chronic health problems that could interfere with a vaccine's protection, for example.

The Centers for Disease Control and Prevention has a long history of following the effectiveness of vaccines after they are approved. On Thursday, the agency posted information on its website about its plans to study the effectiveness of coronavirus vaccines. It will find opportunities to compare the health of vaccinated people to others in their communities who have not received a vaccine.

What exactly are these vaccines effective at doing?

The clinical trials run by Pfizer and other pharmaceutical companies were specifically designed to see whether vaccines protect people from getting sick from COVID-19.

If volunteers developed symptoms like a fever or cough, they were then tested for the coronavirus.

But there's abundant evidence that people can get infected with the coronavirus without ever showing symptoms. And so it's possible that a number of people who got vaccinated in the clinical trials got infected, too, without ever realizing it. If those cases indeed exist, none of them are reflected in the 95 per cent efficacy rate.

People who are asymptomatic can still spread the virus to others. Some studies suggest that they produce fewer viruses, making them less of a threat than infected people who go on to develop symptoms.

But if people get vaccinated and then stop wearing masks and taking other safety measures, their chances of spreading the coronavirus to others could increase.