Whether to get a second booster is nowhere near black and white. Helen Petousis-Harris and Janine Paynter explain why we can't 'boost' our way out of this grey area.
The information about second Covid-19 boosters can be a bit confusing, and while yes, they are useful not all experts currently agree on their best use. Given how fast the Covid-19 situation moves, opinion is subject to change at a moment’s notice on new intel, including ours.
How protected are we with our current Covid-19 vaccines in the age of new variants?
We are actually in pretty good shape, thanks to some super public health decisions that gave us time to get immunologically prepared for the virus. Most people in Aotearoa have been vaccinated with at least two doses of the vaccine, and a while a large proportion have also subsequently been infected, Covid-related deaths have been declining for months globally and remain at an all-time low.
Most people in the world now have some defence against severe disease through vaccination, infection, or both. However, the game and the goal posts keep changing, and complacency would be folly.
The current vaccines were aimed at preventing severe disease and death, and they have saved an estimated 20 million lives already. The first vaccines deployed were effective beyond expectation. They initially prevented infection, so visions of elimination seemed feasible until new variants emerged that were able to evade immunity.
Unfortunately, the current vaccines are no longer very effective at preventing infection with SARS-Cov-2 for a couple of reasons. One is that for most people the amount of time passed since being vaccinated is long enough for the antibodies in the blood to wane. Secondly, the virus has changed sufficiently to dodge some of the protection the vaccines (and/or infection) provided. Fortunately, the long-term immune memory is strong and reasonably robust to variants. While vaccinated people can get infected, generally they will be mildly unwell, and their immune memory enables prompt mobilisation of their immune system that will curb severe disease. The vaccines are still doing what they were originally supposed to do. That is, prevent severe disease and death.
However, there are those in whom the vaccines do not work as well, particularly the elderly and people with a range of conditions that prevent them making a good response to the vaccines. Also, some people are very frail and even a minor infection can be life threatening. It is in these populations that a repeated booster dose of vaccine can be very important.
What can a first booster do for you?
When it comes to the current Covid-19 variants, a single booster (the third dose) makes a big difference in protection against symptomatic infection - for a very limited period. It also further galvanises your immune memory against all the variants we know of so far. Around eight out of 10 people who have received two primary doses of vaccine have reasonable protection against severe disease, and the addition of a booster increases this to nine out of 10.
What can a second booster do for you?
Like the first booster, a second booster stimulates the production of new antibodies that could be useful in providing protection against infection and very mild disease, but as these antibodies wane over a few months, people most likely to benefit from a second booster are the elderly and those most at risk of complications from Covid-19. The available data supports second boosters for these people, but there is little information to indicate much additional benefit in younger people who do not have underlying conditions.
What about the new Omicron vaccines?
Some new Covid-19 vaccine formulations have introduced Omicron variants into the current mRNA vaccines, alongside those targeting the original ancestral strain. These vaccines are called bivalent, and they have been approved for use in many countries including the US, the UK and Australia. While the antibody responses against Omicron are a bit better, the new bivalent Omicron boosters appear unlikely to make a lot of difference compared to the current formations when it comes to keeping people away from hospital.
At this stage of the pandemic, studies for the newer Covid-19 vaccines have involved fewer people than the original formulations and have relied on measuring the immune response between people getting the original vaccine and the new ones. Boosters using either old or new vaccines drove the antibodies back up beyond that seen after the first booster, which is good news. The new vaccines generated slightly better antibody responses against the early Omicron variants than an additional dose of the old ones.
However, it’s worth noting that while it is useful to measure antibodies there are many other immune system ‘actors’ that protect against this virus. As we do not have a magic level of antibody that can tell us if a person is protected against this illness, or not, we really need some real-world information. The important questions are: Are people with these new boosters better protected against infection? Are they better protected against severe disease, death and Long Covid? It takes time to accumulate enough real-world experience to answer these questions.
A team in Australia did some modelling to estimate what the effectiveness of a second booster might be in a population. Modelling has more limitations than real-world experience, but they found in a population where half of the people are protected from infection through vaccination or infection then an updated booster bumped up protection from 86 percent to 90 percent. However, when it came to severe disease the difference was only 1 percent. In short, it looks as though the most useful booster is the one you can get, and there is little benefit in hanging out for a bivalent that is not yet available here if you want protection over the coming months.
So, should you get a second booster?
There are two ways to look at this. One is from the personal viewpoint where we ask ourselves, what is the best option for me? The other viewpoint is what is best for the entire population.
When it comes to individuals, we are all different. The argument for the benefit of a second booster in a young healthy person is very different to the argument for a second booster in a person living in an aged care facility.
But when it comes to considering the entire population, modelling suggested that in this hypothetical population with 50 percent protected, a booster campaign might result in eight fewer hospitalisations per every 1000 people. Which is a lot of people. Of course, the other consideration, which must always be part of the equation, is the potential to do harm. All vaccines have side effects, mostly minor, but also very rarely serious side effects. When the gain is unclear then potential for harm might deserve more weighting on the individual risk/benefit assessment.
If most people are not protected from infection, then a booster well matched to the circulating variants might make a big difference. Conversely, in a population where most people are protected there is little gain.
If the past three years have taught us anything it is that the Covid-19 situation can change on the spin of a dime, and it is not inconceivable that the virus may shift enough to escape most people’s immunity. Under such circumstances, boosters matched to the circulating strain (as the flu vaccine is) will be essential and urgent. We must remain vigilant and be prepared to recognise the benefits of boosters in yet another wave of variants.
We must also accept that this is not a black and white matter, it is one that involves many shades of grey. While most experts agree that boosters are useful, knowing in whom and when becomes more challenging. Sadly, we cannot ‘boost’ our way out of this. Future evidence will bring a lens of clarity, and sooner rather than later if experience is anything to go by. Stay tuned.
