What should I know about Covid-19 Vaccine Efficacy?
We have heard a lot of percentages thrown around in discussions of the new covid-19 vaccines. What do these numbers mean? Which vaccine is best? If I’m vaccinated, what is my risk of catching covid-19? Can I spread covid-19 if I’m vaccinated?
Short summary: All the vaccines are effective enough to put a massive dent in the pandemic, if there is enough uptake. You can’t use the efficacy percentages to compare vaccines and say which is best, and in general these percentages not very accurate or informative on their own. You can probably still spread covid-19 if you’re vaccinated.
When you see a percentage efficacy for one of the covid-19 vaccines, what does it mean?
The first type of immunity that vaccines can provide is called effective immunity, which means the vaccine will stop you getting seriously ill or dying. With all the trials globally with all the different vaccines, the good news is we’ve seen no covid-19 deaths at all from anyone vaccinated in the trials, and only extremely low numbers of people hospitalised, usually a single person out of tens of thousands. All the vaccines have 100% or almost 100% efficacy of this type.
For the majority of us, this is all we need to know. The vaccines work extremely well. However, there are other types of immunity that are also being investigated in the trials, and there have been multiple trials for the same vaccine, which is why there are all the confusing percentages that get thrown around.
Many of the clinical trials are usually using one of these definitions: immunity to severe disease and immunity to any disease. If the trials found lots of vaccinated people caught covid-19 and had symptoms (like fever and fatigue) but nothing more, then that vaccine would be considered ineffective for “immunity to any disease”. We have seen this with the AstraZeneca vaccine against the so-called South African variant, though it is still likely extremely effective against preventing death/serious illness. So an ineffective vaccine is not as terrible as it may sound at first.
The final type of immunity is called sterilising immunity which means the virus is instantly destroyed when it enters your body. It’s very rare for vaccines to provide this level of immunity. Instead you may have this type of immunity if you caught covid-19 and survived, and it normally lasts up to 7 months. It means you have antibodies floating around your system ready to destroy any covid-19 virus on contact before they can invade your cells and replicate. As there’s no evidence for sterilising immunity from covid-19 vaccines this means the virus can probably survive and replicate for a short time in vaccinated people — more on this later.
Usually, when you see a percentage efficacy value for a vaccine, it specifically means “if you give a group of people the vaccine, how many of them will get some symptoms if they catch covid-19”. If you want to know the details about where the numbers have come from, see this article.
So which vaccine has the best efficacy?
In science, you’re not allowed to compare the percentages side by side from different trials for different vaccines. You often see vaccine efficacy percentages compared against each other on news, but you really, really can’t conclude from this that “one vaccine is 10% better than another” or that “this is the best vaccine”. The only way to work this out is to put the vaccines head-to-head in the same trial under the same conditions, and there’s no funding for that kind of trial.
Remember, these trials used different age groups, different countries, different doses, different delays between doses, different health conditions from participants, different numbers of people, and had different levels of covid-19 exposure. You’re not comparing like for like. One important note is that the trials were mostly focussed on questions about severe side-effects of the vaccine and is it having a significant effect on covid-19 disease compared to the placebo. The trials have been successful and they are not useful for asking more exact numbers. Research from countries with large uptake of the vaccine, like research looking at the population of Israel will give us more accurate answers. Research may be able to give some conclusion in several years time about which was the best vaccine, but honestly it doesn’t make a difference.
No really, which is the best vaccine?
If you really want to judge these vaccines based on percentage, then your magic number is 40%. Any vaccine with efficacy above this value for protection against serious disease/death will have a huge impact on the pandemic, given enough uptake (>65% of a population). You may think this percentage is surprisingly low especially as some of the efficacy values from trials for the covid-19 vaccines are above 90%, but that’s because even minorly effective vaccines have a massive impact. They’re one of the best medicinal interventions ever invented. Remember, we already know all the covid-19 vaccines have close to 100% protection for death and serious illness.
So which vaccine is best? The one you get injected with.
If I’m vaccinated, what does the efficacy tell me about my chance of catching covid-19?
The percentages about efficacy are about how the vaccine works in a population. It’s asking for an estimate: if I vaccinate 100 people, how many of them will have an immune system that won’t learn from the vaccine? It’s also not about “95% less covid-19 disease” or “if I’m exposed 100 times, I can expect to get sick 5 times”, the percentages are more like giving you a gist about how it will work in a population.
The efficacy percentage won’t give you much information about how the vaccine will work for you personally. Your personal risk is about how your body responds to the vaccine, so if your body has a strong immune response and subsequent strong antibody response to the virus or not. It’s a bit like how a student’s grades are based on the teacher but also the student. The vaccine gives your body the opportunity to learn how to fight off covid as effectively as possible, but your body still has to “study” and perform in the “exam” when the virus really does show up.
Factors that will influence your body’s immunity mostly include your age, any underlying conditions, the number of viral particles you’re exposed to at once (viral load), if you’re already infected with something else, and then there will be a random element to it as well. You can’t really calculate this but you can guess based on the broad categories how at risk you are (age, what underlying conditions you have, etc). There’s plenty of evidence, however, that even if you don’t get enough immunity from the vaccine to prevent you getting covid-19 disease, then your symptoms will be far less serious.
You can’t calculate your personal risk accurately but you can be part of reducing the overall risk for everyone by getting a vaccine. If you are in the unlucky group where it doesn’t work too well, with enough people vaccinated you won’t encounter the virus anyway, and that’s what we should aim for.
Can a vaccinated person spread covid-19?
The research shows that it is possible to catch covid-19 and even spread the virus a little when you are vaccinated and even when the vaccine worked well for you. However, in this situation you won’t have any symptoms and the virus won’t play full havoc in your body and damage your tissues and organs. You wouldn’t get symptoms. It’s a bit like a small insurgent group being able to set up tents for about 16 hours before they’re obliterated with US air strikes. Technically the virus was there but it didn’t do much.
That being said, it would be beneficial to keep this in mind if you are vaccinated, and still have some level of social distancing until the number of covid cases comes right down in your country/area. You can still be part of the problem even with the vaccine and unwittingly pass it on. It is probably really sensible, for example, when you are allowed to mix closely with other people to take a couple of days off in between. Just in case you did catch it, your body needs several hours for your immune response to kick in. But once covid isn’t spreading around the population we can all act recklessly again (phew).
