Britain and other nations are considering ways to stretch scarce supplies of Covid-19 vaccines, including by delaying second doses, reducing dose sizes and switching vaccine
types between the first and second shots.
The proposals have generated fierce debate among scientists. Here is the rationale behind, and criticism of, these alternative strategies.
In clinical trials, companies tested specific doses of their vaccines at precise time intervals to generate evidence showing how well they work. All Covid-19 vaccines approved, so far, are designed to teach the immune system to recognise and defend against the virus with a first dose, and then provide a second booster dose to reinforce that lesson.
Faced with a fast-spreading pandemic and new, more transmissible coronavirus variants, some countries are hoping to widen immunisation by giving some protection to as many people as possible with a first dose, and delaying second doses.
Maximising the number of people who have partial immunity “should reduce the number of severe Covid-19 cases and thus alleviate the burden on hospitals”, said Michael Head, a global health expert at Britain’s University of Southampton.
None of the late-stage Covid-19 vaccine trials compared these dose-sparing strategies or the effects of mixing vaccine types, said Stephen Evans, a professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine (LSHTM).
Officials have cited limited evidence from trials that the Pfizer/BioNTech, the Oxford University/AstraZeneca and the Moderna vaccines all confer some protection against COVID-19 after the first dose.
Britain’s MHRA health regulator said on Dec. 30 it had found an 80% success rate for the Oxford/AstraZeneca vaccine when two full doses are administered, three months apart, higher than the average that the developers themselves had found.
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Source: IOL
