IT’S hard to believe over the past five months more than half of Scotland’s population has received at least one dose of a Covid-19 vaccine.

That’s a huge achievement and part of the reason why the roadmap for easing restrictions is on track and the focus is gradually shifting from crisis management to recovery. But we’ve always known that one of the hardest to predict elements of the vaccine programme is supply. That’s why contingency planning is important, and why we are relying on a several different vaccines.

This week we learned that the UK has ordered 60 million more doses of the Pfizer vaccine. It has already accounted for four in ten doses delivered in Scotland. The 60 million addition brings the total ordered doses to 100 million – equivalent to the UK’s order for the Astra Zeneca vaccine.

The extra jags have not yet been manufactured but are intended to be ready for a future booster campaign. Considering boosters seems a stretch given that most people under 40 haven’t had their first dose yet, and only one in five Scots has had a second dose. But it’s a next step. So why would we need a third dose, how would it work and who would receive it?

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All vaccines work by supporting our immune systems to recognise and respond to disease-causing organisms including viruses. Many vaccines require more than one dose, with the first one triggering an immune response and then a subsequent dose providing a boost that builds on that. The immune cells ‘remember’ vaccines we have had in the past and respond more rapidly to the second dose. Dosing is based on findings from clinical trials and the approval of a vaccine by regulators such as the MHRA. Although there are Covid-19 vaccines designed to work with a single dose (including from Johnson and Johnson which is likely to be approved for use here in future), both the Astra Zeneca and the Pfizer vaccines have been approved on the basis of two doses.

What we don’t yet know about either vaccine, or the third used here (Moderna) is how long protection after two doses will last. A further booster of the type the extra Pfizer doses are intended to provide could be needed. We heard last month from the chief executive of the pharmaceutical company BioNTech (that partnered with Pfizer to develop their vaccine) that protection declines from 95% overall to around 91% after six months, so a third dose might be needed nine to twelve months after the first to get protection up to higher levels.

Another reason why we might need a booster campaign is if the vaccines have to be modified to respond better to new variants. Evidence from two types of studies: those done in the lab where blood is taken from people with antibodies who have had a vaccine; and studies with people who have been vaccinated, are encouraging even in the face of new variants. But that could change in future and pharmaceutical companies are already developing modified vaccines. Moderna announced last month that it was giving a modified vaccine (designed to respond to the variant first detected in South Africa, the B.1.351) to volunteers in one of its trials. We already offer an annual booster campaign for flu vaccines. Boosters don’t need to go through the same extent of clinical testing and can be approved by medicines regulators quickly.

Who would need the boosters? The UK vaccines minister has previously indicated that the top four priority groups – those over 70, those who are clinically extremely vulnerable and NHS workers and care home staff would be the main beneficiaries. These are the groups most at risk from Covid-19 or of passing it on. Whether we’d need to provide boosters to other groups remains to be seen.

It is also possible that a Covid-19 booster could be offered at the same time as the annual flu vaccine campaign and even offered at the same appointment, which would be complex to set up but easier for patients. Whether this will be recommended still needs to be decided. Another approach could be pre-emptive, which would be to provide a booster to the most vulnerable in August or September, well ahead of winter when there is most concern about a possible future surge in Covid-19 cases.

A final consideration is whether patients receive a third dose of the same vaccine or a different one. For example, would some of the now-ordered 60 million Pfizer doses go to people who had received two Astra Zeneca jags? Mixing vaccines is being tested in the Com-Cov study led by Oxford University. There are some reasons to believe that a mix and match approach with vaccines that stimulate the immune system in slightly different ways might generate greater immunity. Com-Cov will be extended to test a third dose and would inform any roll out.

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All this discussion of boosters is occurring while the pandemic is still raging globally. Some have asked why we are ordering more doses when some countries haven’t even started their vaccine programme and others are struggling to secure supplies.

The UK is a member of COVAX which is the facility for developing, manufacturing and distributing vaccines around the world. The UK Government has indicated that it will donate surplus vaccine supply to low and middle income countries but the timelines remain unclear.

Playing our full part in the international vaccine effort will be essential. This is not just to protect people in other countries, but to also allow us to reconnect with the rest of the world in the months to come.

Our columns are a platform for writers to express their opinions. They do not necessarily represent the views of The Herald.