So, why might we need extra doses of vaccines?
The reasoning for why we might need extra doses of vaccines is clear; an extra dose of the same Covid-19 vaccine will “boost” the immune response created by the first two doses in the hope that it will give longer term immunity.
But added to this is the opportunity for an alternative strategy giving an extra dose of vaccine allows for a reformulated “new” vaccine which is a closer match to newer variants of SARS CoV2 in order to improve the immunity of people against those new variants.
Both are potentially valid reasons.
Is this a “new thing”?
We use booster vaccines all the time in our national vaccination program. Within the childhood program there are lots of vaccines given at 2, 3 and 4 months as well as 1 year that are then boosted. These include vaccines for Haemophilus influenzae type b (Hib), Streptococcus pneumoniae, Neisseria meningitidis group b, diphtheria, tetanus, pertussis, polio, mumps, measles and rubella. That’s a whopping 10 types of infection protected against by booster vaccines already! So, boosters are really nothing new.
You’ll notice I haven’t included influenza vaccines in the booster category, and that’s because they are not boosters. Influenza vaccines are an example of new vaccines against variants of an organism. Influenza Virus mutates frequently, either by antigenic drift or antigenic shift, and so we need new vaccines against the currently circulating variants every year. This is why everyone at risk, including us who work in healthcare, are encouraged to get a flu jab every autumn; it’s because we haven’t yet met the new variant of the Influenza Virus that is coming and so won’t be immune. The flu jab we get is based on a best guess of what we will face due to what is currently causing influenza in the opposite hemisphere.
So, we are used to getting new vaccines all the time as well as boosters, so what is being proposed for the Covid-19 vaccine is nothing new. However what strategy we need for a Covid-19 vaccine is a bit more complicated and it helps to think about what vaccines do in order to choose the best way forward.
What do vaccines do?
Vaccines are a way of cheating nature. Okay, that’s a bit extreme, but it is essentially true. Without vaccines we are likely to get exposed to infectious microorganisms which get into our bodies and then cause disease. During the disease process our bodies react to the microorganisms through our immune systems and we start to mount a defence and try and get rid of the invaders.
There are two main parts to the immune response, the innate and the adaptive systems.
The innate system is a “rapid deployment” response which is pretty non-specific but aims to “hold the invaders in check” until the more specific, but slower, adaptive response can get rid of them. The innate response uses things like interferon to warn nearby cells to prepare to “repel boarders” as well as molecules to attract white blood cells to come and kill the microorganisms as well as the cells they are in.
Whilst the innate system is doing its thing, the adaptive system is slowly ramping up to produce antibodies against the microorganisms as well as other white blood cells such as T cells, which are specifically “created” by the body to recognise and get rid of the invaders. The adaptive system is much better at getting rid of these invaders; it just takes time to get going. Once created, the adaptive response is “remembered” by the immune system so that next time the body comes under attack from the same invader it can go to the adaptive immune response really quickly and kick out the invaders before they cause an infection. The body is immune to the disease. Aren’t we great!
And this is where vaccines help; they short cut the creation of an effective adaptive immune system so that when you first come in to contact with the causative microorganism your adaptive immune system rapidly get’s rid of the invader… you are already immune even though you have never actually had the natural infection. It’s really clever!
But there is a problem with some vaccines… longevity. The vaccine induced adaptive immune response isn’t always long lasting.
The need for boosters
Over time our immune responses to past infections or vaccines can become less effective. We essentially “forget” we are immune. As a result, we can still sometimes get infections even though we have been immunised in the past.
This is where boosters come in. Booster doses of vaccines restimulate the adaptive immune response making it more effective (it also makes the quality of the response better) as well as improving the memory of the immune response therefore making it longer lasting. If we don’t need a new response (e.g. the microorganism doesn’t show a new variant), we don’t need a new vaccine; we just need a dose of the original vaccine to do the trick. It’s the same as getting exposed to natural infection frequently which also boosts the immune response e.g. regular exposure to chicken pox, glandular fever and Cytomegalovirus (CMV), etc. We come into contact with these regularly and our level of immunity stays high.
So, if our immunity against SARS CoV2 wanes over time it is possible that we may need booster doses of Covid-19 vaccines. But there is no evidence of this yet. So far, the immune response to Covid-19 vaccines seems to be pretty long lasting. Very few people have developed Covid-19 symptoms having had 2 doses of vaccine (don’t forget you can still be an asymptomatic carrier!), and the small studies looking at antibody levels post vaccination haven’t yet shown them waning. But, then again, it’s still early days. Maybe over the next few years it will be shown that antibody levels do drop to ineffective amounts… but don’t despair; it’s not all about antibodies!
Remember the T cells!
Remember, the adaptive immune response isn’t all about the antibodies; there are other cells involved as well, especially the T cells. The Covid-19 vaccines produce excellent T cell responses and T cell memory, and that is really effective at preventing future infections. In fact, it’s possibly even better at preventing viral infections than antibodies as T cells target the initially infected cells more effectively and aborts the infection rapidly.
In contrast to the Covid-19 vaccines the influenza vaccines don’t produce a good T cell response, just an antibody response, and this is another reason why we need such frequent influenza vaccines. The influenza vaccines just aren’t that effective over the long term… but maybe vaccine manufacturers will learn from the Covid-19 vaccine program and we will see some new and improved influenza vaccines in the future. I guess that might come down to that old favourite “money”, surely it is economically better for the manufacturer to sell new vaccines every year rather than developing long term effective “one off” jabs?!?
On top of the robust T cell response from our double vaccine, we are also likely to get natural boosting of our Covid-19 immunity. There is a lot of natural Covid-19 exposure going on at the moment… over 30,000 new cases a day in the UK and who hasn’t been “pinged” by track and trace?! This means that as the restrictions on life reduce next week we are all more likely to come into contact with the natural infection, which with luck will boost our immune systems to be more effective without the need for further vaccines... at least I hope so…
Over time this natural boosting is what will happen anyway. It is what happens with the common cold coronaviruses already. We come into contact with these viruses on a regular basis, get a bit of a cold and an immune top up, and we carry on with daily life without even realising it. This natural boosting is the likely future of Covid-19… in time that is!
What do I think we should be doing?
I have my own view on the use of vaccines as boosters. I think we would be better off, both morally as well as scientifically, donating these doses of vaccine to countries who are struggling to control Covid-19. It is in our own interest to see the whole World immunised as quickly as possible. Are we better off having a small number of people vaccinated 3 times, or lots of people vaccinated at least once? Making sure Covid-19 is controlled all over the World is the best strategy for protecting everyone, including those of us in high vaccination countries, after all that is what we did for smallpox, and what we are trying to do for polio.