Analysis: time, data and new variants will have a heavy influence on whether Covid vaccine boosters are required in the future

We have never spoken so much about our immune systems and vaccines as we have in the last 18 months. It has been a hot topic in the media and our views about vaccines have crept into our everyday lives and a common part of our daily conversations now includes the question 'did you get your vaccine yet?'.

Ireland has done a phenomenal job with the Covid-19 vaccination roll out, which has ramped up in line with supply over the last six months. We started with around 40,000 doses a week and are now at approximtaley 350,000 doses a week. We are the envy of Europe for both the speed of our vaccinations as well as the vaccine uptake in our population. The emergence of new variants of Covid-19 - the Delta variant, in particular, which can evade vaccines - has caused concern for the vaccine programme and raised the question of whether we will require booster shots in the future to keep us protected. Is it too early to know the answer?

How does your immune system respond to a viral vaccine?

Viral vaccines work by imitating the natural infection and so they expose your immune system to part of the virus that causes the infection, without causing the infection itself. Your immune system doesn't realise the difference and responds by switching itself on as if it were fighting the infection. This results in two key events – the production of antibodies and the activation of immune cells called T cells and B cells. These provide a good level of immunity against the virus.

From RTÉ Radio 1's Today With Claire Byrne, Prof Christine Loscher on the vaccine rollout in Ireland

If you are exposed to the virus following vaccination, the antibodies stick to the virus so it can’t infect your cells. The T cells kill cells with virus inside them and the B cells become factories for making more antibodies to join the fight. Very cleverly, your immune system makes memory T and B cells, so it "remembers" the virus the next time it comes into contact with it and is ready to fight. Memory B cells are very important because they stay in your body and will still be able to make antibodies years later if you are exposed to the virus.

Single and double shots – why?

While a single vaccine shot can provide good immunity to a virus, most vaccines require a second shot so the second shot is essentially a booster. A quick look at our childhood vaccination programme in Ireland will very clearly demonstrate that most vaccines need two shots. This can be for a few reasons:

- Often the first dose of a vaccine does not provide as much immunity as is possible, therefore a second shot is given to maximise immunity. Examples include the Hib meningitis or the MMR vaccine.

- For some vaccines, the immunity level decreases with time and a second shot is needed to boost immunity. A good example here is a tetanus vaccine.

- In some cases the virus changes, such as the flu virus and shots are required every year to protect against seasonal strains.

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From RTÉ Radio 1's Morning Ireland, The Irish Times Health Editor Paul Cullen on the race against time to get as many people as possible vaccinated before Ireland is hit by a fourth wave.

What does the second shot do?

The immune response to a second vaccine dose is even more powerful than the first. After the second shot, the numbers of T and B cells as well as antibody levels are significantly enhanced. Furthermore, the antibodies are more powerful and are better at binding to the virus to stop it from invading your cells. More memory B and T cells also mean that your immunity will last longer.

It's already very clear that booster shots are critical against the new Covid-19 Delta variant. The first dose of the AstraZeneca vaccine confers 30% protection against infection, with the second shot increasing this to approximately 60%. The first dose of the Pfizer vaccine provides approximately 36% protection against infection, with the second shot increasing this to almost 80%. The effect of the second shot is very clear.

What do we know so far about Covid-19 immunity?

We know that immunity to viruses can last for a very long time, both after the natural infection and following vaccination. For example, some people who got infected with SARs-COV-1 in the 2003 epidemic still have immunity 17 years later. Even more incredulous is the fact that some people who got the flu in 1918 still had some immunity some 90 years later. Estimates on how long immunity lasts with common place vaccines, such as measles, mumps and whooping cough, are anywhere between 10 and 70 years depending on the vaccine. Immune memory is very powerful.

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From RTÉ Radio 1's Today With Claire Byrne, Prof Christine Loscher and Dr Yvonne Williams GP on children and Covid vaccines

The evidence from people who had natural infection with Covid-19 tells us that immunity is still present after nine to 12 months. As time moves on, we will know if this immunity persists for longer. We expect that it will, but we have to be careful not to just look at antibody levels as memory T and B cells are important too. Some studies show that while antibody levels to Covid-19 may decrease within a few months that potent memory B cells remain and can make these antibodies when required. This might suggest long lasting immunity.

The big question: will we need more booster shots?

Recent research looking at the AstraZeneca vaccine showed that a third dose of the original vaccine more than six months after the second dose induced a strong boost to immunity to Covid-19, including the Alpha, Beta, and Delta variants. This suggests that boosters will be very beneficial for dealing with variants. Another study showed that vaccination after recovering from the natural infection was significantly higher than just vaccination alone and it is expected that this will provide long lasting immunity and these individuals may not even need boosters.

If we look to a country with a very advanced Covid-19 vaccination programme such as Israel, they have recently stated that they are considering boosters as they think that immunity is decreasing in their population. One explanation for this may be due to the decreased effectiveness of vaccines against the Delta variant.

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From RTÉ Radio 1's Drivetime, Prof Kingson Mills on whether we need need a booster shot in the race against new variants

The World Health Organization has said it was not clear whether Covid-19 booster vaccines will be needed to maintain protection until further data is collected. However Pfizer have announced that they will seek regulatory approval for a third dose of their Covid-19 vaccine. A key driver for this was the real world data coming from Israel, but also some initial data from a trial that showed a third shot significantly enhanced antibody levels further than 2 shots. If Israel administers a third dose to their population in the very near future, it will provide interesting real world data and may answer some questions about the need for Covid-19 vaccine boosters.

'Time will tell how much Covid will impact our everyday lives'

The Covid-19 vaccine programme is in its infancy. In order to know if we will need boosters, there are three key things we need to know – and these will only come with time. First of all, time will tell us whether immunity from the vaccine will be long lasting. Seconly, we will understand more about the Sars-Cov2 virus and know whether it will require yearly seasonal vaccines similar to the flu vaccine programme.

Thirdly, the emergence of variants will have a heavy influence on whether vaccine boosters are required, in particular with vaccines which have been modified slightly to boost immunity to particular variants. The good news is that we have the technology to do this. There is no doubt we will be living with Covid-19 well into the future, but only time will tell how much it will impact our everyday lives.


The views expressed here are those of the author and do not represent or reflect the views of RTÉ