Several Indian states have reported an acute shortage of vaccines, forcing many regions, including the capital New Delhi, to again prioritise those aged above 45.
A health official said the trial will include around 500 military personnel who were vaccinated with a first dose of AstraZeneca's COVID-19 shot.
The study will examine T cells - immune cells that can destroy virus-infected cells - and neutralising antibodies in those who were given a combination of doses, the health official told reporters.
Scientists started studying data from the day Italy's vaccination campaign began, on Dec. 27 2020, until May 3 2021.
As of Saturday morning, some 8.3 million Italians, or 14% of the population, were completely vaccinated, while around 10 million people had received a first jab.
This is because available stocks amount to just 529,000 doses of Pfizer's vaccine and 381,000 doses of the AstraZeneca product, an agency official told Reuters.
A surge in recent days has seen patients' families taking to social media to beg for oxygen supplies and locations of available hospital beds, and has forced the capital New Delhi to extend a week-long lockdown.
Optimism nevertheless appeared intact among traders as bitcoin zoomed to a record high above $63,000 ahead of Wednesday's trading launch of the Coinbase cryptocurrency exchange.
Nearly 5 million people in the United States had received J&J's vaccine as of Thursday morning, according to the U.S. Centers for Disease Control and Prevention.
Europe's medicines regulator said this week it found rare cases of blood clots among some adult recipients of the shot, although the vaccine's advantages still outweighed its risks.
Greece has reported 288,230 cases of COVID-19 and a total of 8,680 deaths. It has administered 378,997 doses of the AstraZeneca vaccine so far and ordered another 1.35 million doses.
Mexico's drug regulator Cofepris said on Wednesday it has no current plans to limit the use of the Oxford/AstraZeneca COVID-19 vaccine to any age or group.
"Regulatory authorities in the UK, European Union, the World Health Organization have concluded that the benefits of using our vaccine to protect people from this deadly virus significantly outweigh the risks across all adult age groups," a spokesman for AstraZeneca said in a statement.
"AstraZeneca confirmed to us not being under any obligation to other parties that would impede to complete the fulfilment of its obligations" to the EU, the Commission spokeswoman said when asked about Hancock's statements.
Under the EU contract, AstraZeneca committed to supplying vaccines produced in four European factories, two of which are in Britain: Oxford Biomedica and Cobra Biologics.
He added that he expected to be able to get 10 million doses of vaccines each week in May but that the Olympics, which a majority of Japanese have said should be postponed or cancelled, were not a factor in his scheduling.