A study by Oxford University showed delaying second and third doses of the AstraZeneca COVID-19 vaccinestill provided strong protection against the illness. The study was published in pre-print, which means it has yet to be peer-reviewed.

The university's study, released on Monday, said an interval of up to 45 weeks between the first and second dose enhanced immunity, rather than compromised it.

"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 study lead investigator Andrew Pollard.

The researchers also said the findings on a potential third dose, which might be used as a booster, were positive. The study's lead senior author, Teresa Lambe, said "it is not known if booster jabs will be needed due to waning immunity or to augment immunity against variants of concern,” but said the results were good "if we find that a third dose is needed."

The AstraZeneca vaccine is currently administered in 160 countries. It was hailed for its low cost and ease of refrigeration and transportation. However, confidence in the vaccine fell after concerns to links to extremely rare, but serious blood clots.

Mixing vaccines appears to work

In a separate Oxford study, researchers found that mixing Pfizer-BioNTech and AstraZeneca vaccines in two-dose schedules produced high amounts of antibodies against the virus' spike protein.

The scientists looked into different responses triggered by giving patients two doses of Pfizer-BioNTech's jab, two doses of the AstraZeneca vaccine, and by combining the products. The study, labeled Com-COV, measured both antibody and T-cell responses. While antibodies are responsible for preventing the virus from infecting cells, T-cells search and destroy cells that have already been infected.

The researchers found that the two doses of Pfizer-BioNTech provided the strongest antibody response, but combining the product with one dose of AstraZeneca was still more effective than administrating two doses of AstraZeneca. An AstraZeneca vaccine, followed by a Pfizer-BioNTech dose produced the best T-cell response.

The trial involved 830 participants given vaccines at four-week intervals. It has also been published in pre-print.

The university, which worked with AstraZeneca pharma company to develop the vaccine, said the findings could give flexibility to vaccine rollouts. At the same time, they warned that the findings were not significant enough to recommend a shift away from previously approved schedules.

"It's certainly encouraging that these antibody and T-cell responses look good with the mixed schedules," said Matthew Snape, an Oxford University professor who was involved with the trial.

"But I think your default has to stay, unless there's a good reason otherwise, to what is proven to work,” he added.

kbd/dj (AFP, Reuters)