 Different COVID vaccines have different efficacy. Efficacy is basically how less likely is the infection in a vaccinated person when exposed compared to a person who is not vaccinated. Percentage or proportion of people who get infected when exposed is known as attack rate. For example, suppose 100 people are vaccinated and 100 people are not vaccinated and say suppose only one person out of 100 who are vaccinated gets the disease while 20 people who are not vaccinated get the disease. So we say attack rate in vaccinated group is number of people infected divided by total number of people vaccinated. So in this case it will be 1 by 100. While that in non-vaccinated group is number of people infected divided by number of total non-vaccinated people. So it will be 20 by 100. Now to calculate vaccine efficacy we determine something known as relative risk reduction. This is basically a ratio of attack rates in people with and without the vaccine. So to calculate relative risk reduction we divide attack rate in vaccinated with attack rate in non-vaccinated. So in our case it will be 1 by 100 divided by 20 by 100. So this comes to 1 by 20. That means a vaccinated person is 20 times less likely to get the disease when exposed. Now from this relative risk reduction we calculate vaccine efficacy by this formula. 100 into 1 minus attack rate in vaccinated divided by attack rate in non-vaccinated group. Which can also be written as 100 into attack rate in unvaccinated minus attack rate in vaccinated divided by attack rate in unvaccinated. So this value we know is 20 minus 1. So in calculation it will come to 100 into 19 by 20. That is the 5 into 19 is equal to 95 percent. So that's how we calculate vaccine efficacy. Now for covid we have different vaccines with different efficacy and all of them have been approved. This table shows various approved covid-19 vaccines and their efficacy. Pfizer has an efficacy of 95 percent, Moderna an efficacy of 94 percent, Sputnik V of 90 percent while Johnson and Johnson and AstraZeneca Covid shield have an efficacy of 67 percent. There are various factors which affect the efficacy of vaccines. That is a subject of another video perhaps. However I have a question. But how come these vaccines which differ from each other so much on efficacy all of them have been approved? So why it has been done like that that even vaccines with lower efficacy have been approved? See there is something known as efficacy threshold. That means an upper limit of efficacy which the vaccine should have to get it approved. This depends on the disease. For covid vaccines this efficacy threshold was kept low at around 50 percent only because the disease is severe and so many people are dying because of the disease. So despite the efficacy being lower these vaccines were approved. Since there was a desperate need of something to prevent the spread of the disease and people from dying. So it was still worth using vaccines of lower efficacy. Second thing also important was that if only one vaccine with higher efficacy was approved then maybe the production time would be limited then there would be only one formula by which vaccine is being produced. So it would be difficult to distribute to so many countries across the world. So that's the reason that why vaccines with different efficacies have been approved. Well thanks for watching the video. If you liked it do press the like button share the video with others and don't forget to subscribe to the channel Physiology Open. Thank you.