COVID vaccines: efficacy explained

At the time of publishing, around fifty countries worldwide - albeit none in Africa, South Asia, or Australasia - have begun administering a variety of COVID-19 vaccines. That is, by all accounts, astonishing progress. So are we in the home stretch? Is everything going to plan? Or could something still go drastically wrong? Viral immunologist Zania Stamataki helped take apart some of the specifics behind these questions - in an interview with Phil Sansom that first aired on the Naked Scientists podcast...

Zania - Well, aren't we fortunate. We've got over 200 vaccines in development, a handful already approved in different parts of the world. The data from clinical trials are pouring in and guess what? The vaccines work. We now need to get the logistics right, to protect our vulnerable and then the rest of us. And it's important to note of course that while vaccinated, although we are protected, we may still transmit the virus to others.

Phil - This Oxford vaccine - can you remind us, what exactly is it?

Zania - The Oxford vaccine has used a harmless chimpanzee virus to infect our cells and pass on a message, the genetic information, so that we can make coronavirus spike protein ourselves. This stimulates our immune system and we prepare our defenses, which takes a couple of weeks to happen. And after that, our body's ready, and if infected with the coronavirus, our immune system can stop it in its tracks before we get COVID.

Phil - We've got a question for you about it as well, from listener James who asks: "Can you explain the Oxford COVID vaccine approval by MRHA, the UK regulator and what new data is now available since their," and I quote from him, "horrendous first reporting in early December? Seems as though the UK population is being offered a 62% minimally effective solution to me." What is he talking about and what is your opinion?

Zania - Well first of all, we need to separate the vaccine effectiveness from efficacy data. Efficacy is the 62% number that your listener was quoting. And he's talking about the outcomes of clinical trials that are very short and contain a small number of people. Now don't get me wrong, the vaccine has been given to thousands and thousands of people before the clinical trials concluded, but only a small proportion of these people have become infected. So we are getting data back from a small amount of people. Now the Oxford vaccine was given to people in two different doses; some of them showed 62% efficacy in that arm of the trial; others that received a lower dose at the beginning of the trial showed up to 90-odd percent efficacy, which is all very good. But let's remind ourselves that the FDA had said that they will approve any vaccine with efficacy above 50%. And for flu vaccinations, we accept vaccines from 20 to 60% efficacy every year. So the Oxford vaccine we expect to be highly effective, and the effectiveness data is going to change as the vaccine is rolled out and we get data from real life people.

Phil - What would you say to someone who said, “no, I want the Pfizer vaccine, that one's got a higher number. That looks better to me."

Zania - We can't really make that decision based on the data that we have because we haven't compared vaccinations side-by-side, and as scientists, that's how we make comparisons: we get the same population, we vaccinate them with the same preparations, and then we expose them to the disease and we get data back. So this has not happened. And what I can say to people that are worrying about which vaccine is going to be more effective is, I would personally accept any vaccine that was given to me that was approved by MHRA - and I'll be grateful for it as well! I think a vaccine that is approved, that works, is tremendous news. We, as a population, are going to respond differently; but if the vaccine works, take it.

Phil - There are other concerns arising that I'd like to address with you. People are a little bit worried about the UK delaying the second dose of the Pfizer vaccine to people who've received the first one; the manufacturer said three weeks, the UK is saying could be more like months. What's behind that, and is that scientifically sound?

Zania - Well as scientists that work in the lab, we get very nervous when we deviate from a protocol. And Pfizer has only tested their vaccine with a booster within the three weeks, like you said. The Oxford vaccine was tested with later booster jabs too. Now we know however from experience that vaccination boosters continue to work well after several weeks. In the UK we have an urgent public health need, with one in 50 of us currently infected, and we are losing around 900 people a day to COVID. This decision was made to save more lives. But it is important for us to gather data to inform future vaccination protocols for different patient groups.

Phil - One other thing from listener Richard who asks: "What I'd like to know is, what are the risks of developing a serious disease if you catch COVID on the same day as having the first dose of a vaccine? Is there any reduction in your risk of serious disease?"

Zania - Within the day that you are vaccinated, you have not had enough time to generate protective immune responses. As a rule of thumb, you calculate about a week until your responses start to kick off. And then beyond that up to two weeks, when you're mounting decent memory cells, that will protect you from re-infection. So within the first couple of weeks, I would say, take utmost care. You are not protected yet - and this is in your little pamphlet as well that you receive when you become vaccinated - so it takes a little while for the immune system to develop immunity. And this is the whole idea behind vaccination: by getting our jab, we are giving our immune system a sort of stimulation early on so that when we come into contact with the real thing we'll be good to jump to it.

Phil - Zania, if I can ask you to make some predictions for me, how long do you see this pandemic lasting?

Zania - Well it depends on ourselves, really. It depends on our personal behaviour. You do need the vaccine to generate immunity for a population, so that we can protect each other for a long period of time. But if you keep to the rules, as they have been advised to: mask yourself so that you can protect others from infection if you are asymptomatic; keep your distance, so you don't catch the virus yourself; wash your hands; avoid touching your face, like I do all the time without realising - I've become so used to my sanitiser now because I just can't stop touching my face! If you stick to the rules you can protect yourself. And in fact we know from countries around the world that have been very, very strict at sticking to the rules, that they can control infection. And this way, when you have outbreaks that are small, they can quash them very, very quickly. So it is possible for us to have good news, even in the absence of vaccination; but for us to eradicate the virus as a problem, we do need to vaccinate ourselves.