Coronavirus: what is the science behind UK plan to delay second dose of vaccines?

Governments and health experts worldwide are considering whether to follow the UK in inoculating as many people as possible against Covid by giving them each just one dose rather than two.

The debate has been raging since the end of December, when Boris Johnson’s government announced plans to postpone giving the second dose of both the Pfizer/BioNTech and the Oxford/AstraZeneca vaccines by up to 12 weeks.

Denmark has now unveiled plans to delay the second dose of both the Pfizer and forthcoming Moderna jabs by up to six weeks, and Germany is considering a similar move.

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But some experts are warning that “delaying could cause further virus mutations and render the shot ineffective”, while others “question whether recipients will be left more vulnerable”, says Politico.

“The only thing that is clear is that a second vaccination is absolutely essential because it triggers the necessary immune response, like a kind of booster,” adds Deutsche Welle.

The World Health Organization has “said it understood why a country facing the sort of increases in cases, hospitalisations and deaths that were happening in the UK might decide to go beyond the evidence” on administration of second doses, The Guardian reports.

But there is “very little empiric data from the trials that underpin this type of recommendation”, said Dr Joachim Hombach of the UN health agency’s strategic advisory group of experts on immunisation.

According to the newspaper, “there is some evidence from trials of the Oxford/AstraZeneca vaccine that a late second dose, up to 12 weeks, does not interfere with the efficacy of the vaccine”.

But rival vaccine maker Pfizer has said that no data from its own clinical trials was available “to demonstrate that protection after the first dose is sustained after 21 days”.

Despite the misgivings, some experts believe the gains of delaying vaccine doses may outweigh the risks.

Thomas Mertens, from Germany’s public health agency, the Robert Koch Institute, has come out in favour of the UK plan. “Since the interval between the two vaccinations can very likely vary within wide limits and protection is already very good after one shot, it is certainly worth considering giving preference to the first injection in the event of a vaccine shortage,” he said.

The UK’s Joint Committee on Vaccination and Immunisation, an independent body that advises United Kingdom health departments on immunisation, reports that short-term vaccine efficacy from the first dose of the Pfizer-BioNTech vaccine is calculated at around 90%, and at around 70% for the AstraZeneca vaccine.

Given those high levels of protection, “models suggest that initially vaccinating a greater number of people with a single dose will prevent more deaths and hospitalisations than vaccinating a smaller number of people with two doses”, the committee’s experts argue.