Pfizer's coronavirus vaccine: how it works

The US pharmaceutical giant Pfizer and the German manufacturer bioNTech claim to have created a coronavirus vaccine that has "more than 90% efficacy". Their vaccine is currently in the final stage - Phase 3 - of the approval process; it’s being tested on 43,500 people in six countries; and, so far, it appears to be safe. Nevertheless, despite the hype and hope, there is still an enormous amount we don’t know about this vaccine, which works in a totally new way that’s never been tried before in humans. Anne Moore is an immunologist from University College Cork, and explained to Chris Smith...

Anne - The vaccine works by delivering a piece of genetic material into the cell. And we've been doing this since, I suppose, the mid 1990s with DNA, which is one type of genetic material; and we've always wanted to use the other type of genetic material ,which is RNA. But the big problem with it up to a few years ago was that RNA is so unstable, and it degrades very quickly as soon as you put it into the body, and it's quite difficult to get it into a cell as well. So we had to overcome those problems. So this company BioEnTech - and other companies out there as well, Modern in the US are also taking a similar approach - have changed the RNA to make it much more stable, so it doesn't degrade as easily; and they've also found this little lipid drop that you put the RNA in, so that it doesn't break down when you put it into the body, and then there's enough instructions on that piece of genetic material to drive production of the spike protein from the SARS-CoV-2 virus that causes COVID-19.

Chris - So you're putting in the genetic message corresponding to the outer coat, this spike protein, of the virus; and cells can actually read that genetic message when you inject it, and then make their own version, as though they've been infected by the virus for real?

Anne - Exactly. And the nice thing is that they haven't been infected by the virus for real, or any other virus, so the immune system can just focus on looking and responding to that spike protein from the SARS-CoV-2 virus.

Chris - Admittedly it's early days at the moment, but what do we know about the response the body is making in people who have received this vaccine and vaccines like it?

Anne - Pfizer and BioNTech published some of their phase one data, and what we know is that when it's used, you get high levels of antibodies and very strong T cell responses, which are these cells that can go around and they're like the marines of the immune system: they can kill any virus infected cells, and they can also support the correct development of antibodies as well. So it seems to be a global response overall. What we don't know very well is how much of these responses are needed to protect you from infection from SARS-CoV-2.

Chris - And how long do those responses persist for? Because that's another key question, isn't it? We can't presumably have much knowledge yet, because we've only known about the virus for under a year, and we've only had this vaccine trial going on for a matter of months.

Anne - Exactly. It is an absolutely crucial question: how long can those immune responses, those antibodies and those T cells, remain strong enough in the body to provide that protection? And unfortunately, we're not smart enough to know yet, as soon as we immunise somebody, how long that protection will last. Time is needed to see how long those immune responses stay high. I suppose the other really, really key question that we don't know at all is: what is the threshold of protection that's required? So if you need a very little antibodies or T-cell to still protect you against infection, we have more faith that these vaccines will protect you for longer; but if you need to maintain very, very high levels of antibodies and T cells, then the likelihood is that the durability of protection will wane, will you go down a bit faster than we would like.

Chris - There was enormous fanfare around the 90% number - the announcement was, "it's 90% effective" - but if you turn that round, it means it's a 10% failure, or a failure 10% of the time. But we weren't told who the 10% it doesn't protect are! And if they end up being the same 10% as the people who are destined to get severe illness with coronavirus, arguably it doesn't move us very much further forward, this vaccine.

Anne - So the 90% efficacy means: of the people that were infected, 90% were from the placebo group and only 10% were vaccinated. So it's saying that if you have the vaccine, you have a 90% less possibility of being infected. And for a lot of vaccines, that's more than sufficient. For some vaccines like measles and things like that we do need a higher protection such as 95%. I don't know of any vaccines that will give you a hundred percent protection. Plus as long as 90% of the community are protected, it provides a good barrier for the population. For somebody who works in vaccines, 90% protection is good. We're happy with that. It's a good response.