US health agencies have recommended pausing the use of Johnson & Johnson's COVID-19 vaccine as the company said it will delay its roll-out in Europe while several Muslim-majority countries have announced curbs for the holy month of Ramadan.
European regulators said last week they were reviewing rare blood clots in four recipients of the Johnson & Johnson (J&J) vaccine in the US.
Acting US Food and Drug Administration Commissioner Janet Woodcock said on Tuesday the agency expected the pause - after six women under 50 developed rare blood clots after receiving the shot - to be a matter of days and was aimed at providing information to healthcare providers on how to diagnose and treat the clots.
The moves come after European regulators said earlier this month they had found a possible link between AstraZeneca's COVID-19 vaccine and a similar rare blood clotting problem that led to a small number of deaths.
Immunology experts echoed US officials in underscoring that the risk posed by the J&J vaccine appeared extremely low and it remained a valuable tool against the risks of COVID-19.
The FDA said there had been one reported death from the rare blood clotting condition among recipients of the J&J vaccine while another person was in a critical condition.
FDA official Peter Marks said it was "plainly obvious" the J&J cases were "very similar" to the AstraZeneca ones.
However, officials said there had been no similar blood clot cases reported among recipients of the Moderna or Pfizer-BioNTech vaccines, which use a different technology.
The J&J and AstraZeneca vaccines both use an adenovirus vector - a harmless cold virus - to deliver instructions for human cells to produce a protein found on the surface of the coronavirus, spurring the immune system to recognise and attack the actual virus.
Among leading global COVID-19 vaccine developers, China's CanSino Biological and Russia's Gamaleya Institute with its Sputnik V vaccine also rely on this approach.
The Pfizer-BioNtech and Moderna vaccines use messenger RNA (mRNA) technology.
Muslims in many parts of the world marked the start of Ramadan on Tuesday but a spike in coronavirus cases in several countries has once again put curbs on the holy month's signature feasts and lengthy prayers in mosques.
Still, there were glimmers that Ramadan 2021 could feel less restricted than last year, when Islam's holiest period coincided with the start of the coronavirus pandemic.
Mosques have since reopened and limits on movement have eased as vaccine rollouts continue in Muslim-majority countries.
Clerics in such places as Indonesia have issued assurances the vaccine does not break one's daytime fast.
Ramadan is marked by longer prayers, dawn-to-dusk fasting and nightly feasts with family and friends, though crowded gatherings in mosques and large gatherings for meals remain prohibited.
In Mecca, home to the Kaaba - Islam's most sacred site - Muslims performed socially distanced "taraweeh" prayers marking the start of Ramadan.
Observant Muslims around the world pray toward the Kaaba five times a day.
Only limited numbers of worshippers were being allowed inside the Grand Mosque that houses the Kaaba to prevent the spread of the virus.
Saudi authorities were only allowing individuals who have been vaccinated or recently recovered from the virus to perform taraweeh prayers at the Kaaba.
In Iraq, a curfew will remain in place from 7pm to 5am throughout Ramadan, with total lockdown on weekends.
The health ministry warned that non-compliance with these measures could lead to three-day continuous lockdowns.
Turkey says all sports venues will have to shut during the month of Ramadan and weddings and other events are banned.
Vaccinations pose a challenge for Muslim countries administering shots throughout Ramadan.
Officials were working to ease concerns over the Islamic teaching that Muslims should refrain "from anything entering the body" between sunrise and sunset.
Indonesia's top clerical council went so far as to say Muslims eligible for vaccinations are "required" to take the shots during Ramadan.
In India, where infections have peaked in recent days, scholars are appealing to the country's 200 million Muslims to follow anti-virus protocols and refrain from large gatherings.
Many Indian cities dealing with virus surges have imposed night-time curfews and it remains unclear whether the faithful will be allowed to perform taraweeh prayers in mosques.
In Egypt, the government prevented mosques from serving free meals during Ramadan and banned traditional charitable iftars that would bring together strangers at long tables.