The AstraZeneca COVID-19 vaccine produces an improved immune response with a longer dose gap of up to 45 weeks, and a third shot is able to boost antibody levels even further, according to a study in the UK.

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The yet-to-be peer-reviewed study shows that antibody levels remain elevated for at least one year following a single dose of the Oxford-AstraZeneca vaccine, known as Covishield in India where the gap between its two doses has been set at 12-16 weeks. The authors of the study noted that an extended interval of up to 45 weeks, or about 11 months, between the first and second dose of the AstraZeneca vaccine, resulted in up to an 18-fold increase in antibody response measured 28 days after the second dose. The research, posted on the preprint server of The Lancet on 28 June, included volunteers aged 18 to 55 years who were enrolled in the trials and had already received either a single dose or two doses of AstraZeneca vaccine.

The researchers from the University of Oxford in the UK assessed immune response after a single dose, immunity after an extended interval between the first and second dose, and the response to a third dose as a late booster. With a 45-week dosing interval between the first and second dose, antibody levels were four times higher than with a 12-week interval. The finding demonstrates that a longer interval between the two doses of the vaccine is not detrimental but can derive stronger immunity, the researchers said. This should come as reassuring news to countries with lower supplies of the vaccine, who may be concerned about delays in providing second doses to their populations, said Professor Andrew J Pollard, chief investigator and director of the Oxford Vaccine Group at the University of Oxford.

There is an excellent response to a second dose, even after a 10-month delay from the first, Pollard said in a statement. The researchers noted that some countries are considering administering a third booster dose in the future. Studying the impact of a booster, they found that the dose given at least six months after a second dose boosted antibody levels six-fold and maintained T cell response. T cells are one of the important white blood cells of the immune system and play a central role in the immune response. A third dose also resulted in higher neutralising activity against the Alpha, Beta, and Delta variants, according to the findings of the study.

The Delta variant was first reported from India and is believed to have driven the devastating second wave in the country. "It is not known if booster jabs will be needed due to waning immunity or to augment immunity against variants of concern, said Associate Professor Teresa Lambe, lead author of the study. Here we show that the third dose of ChAdOx1 nCoV-19 (AstraZeneca vaccine) is well tolerated and significantly boosts the antibody response. This is very encouraging news if we find that a third dose is needed, Lambe said.

The authors also noted that both the late second dose and the third dose of the AstraZeneca vaccine showed fewer adverse reactions than the first dose. However, they noted that further research is required to follow up with study participants who received third doses beyond the period of the initial study.