Earlier this summer, countries with high vaccination rates were ready to turn a corner in the battle against the COVID-19 pandemic. But the spread of the highly contagious Delta variant in recent weeks has dampened that prospect and forced governments to reintroduce preventive measures.

To safeguard their return to normalcy, several countries that had already lifted most COVID-19 restrictions are starting to offer a third dose of the vaccine, disregarding a call from the World Health Organization (WHO) on Wednesday to halt booster shots until more countries could catch up with vaccination coverage.

Who are getting booster shots?

On Sunday, Israel started administering third shots to people over the age of 60. The country was among the first to embrace normal post-pandemic life after vaccinating more than 70 percent of its population earlier this year.

But with more "breakthrough" cases reported since the Delta variant became widespread, Israeli health experts believe the immunity offered by the original shots declines in people vaccinated over five months ago.

A similar situation is seen in Britain, which reopened its economy on July 19. Starting September, 32 million vaccinated Britons will be offered additional shots.

Germany and France, both with around half their population fully vaccinated, will give booster shots to elderly and vulnerable people from September.

Meanwhile, other countries with the ability to do so, including Japan, Canada, Switzerland and Singapore, are reportedly planning such a move in winter or early next year.

Before the Delta variant threatens to break through the immunity defense achieved through large-scale vaccinations, booster shots were already available in Israel, the United Arab Emirates and some European countries, mainly for people with compromised immune systems.

The science is still murky

The recent spikes of breakthrough infections have raised questions about waning immunity of COVID-19 vaccines, compounded by fears of new variants like Delta, which is responsible for the majority of new cases. But due to a dearth of clinical data, it is unclear how much a third shot will do in preventing illnesses.

Researchers say the protection of the standard two shots lasts at least six months and possibly much longer. The U.S. Center for Disease Control (CDC) and the Food and Drug Administration (FDA) said a in joint statement that fully vaccinated Americans do not need a booster shot at this time, though the health regulators have indicated that a third shot may be needed for some high-risk groups.

Citing its non-peer-reviewed study last week, Pfizer suggested that a third dose of its COVID-19 vaccine can significantly increase protection against the Delta variant. The company said more studies on the third dose are underway and once deemed necessary, it would apply for emergency use authorization for the shot as early as this month.

Other vaccine makers also supported the idea. Moderna co-founder Derrick Rossi even went as far as saying "it might not be surprising that we need a booster shot every year."

The scientific case for a booster shot is based on findings which showed a significant drop in antibodies within a few months of receiving a second dose of vaccine. But scientists also don't know exactly what levels of antibodies are enough to provide protection from the virus.

WHO calls attention to vaccine inequality

While some rich countries are rolling out third shots, many poorer countries are struggling to procure enough jabs for their people to begin.

High-income countries administered around 100 doses for every 100 people, whereas low-income countries have only been able to provide 1.5 doses per 100 people due to lack of supplies, according to WHO Director-General Tedros Adhanom Ghebreyesus, who appealed to wealthier countries to help address the huge disparity and ensure that at least 10 percent of the population in every country is vaccinated.

The UN health agency has argued that no one is safe until everyone is safe, because as the pandemic drags on, more dangerous new variants could emerge and hamper the progress made elsewhere, thus set the whole world back on the road to recovery.

"As we've seen from the emergence of variant after variant, we cannot get out of it unless the whole world gets out of it together," said Dr. Bruce Aylward, a special adviser to Tedros. "And with the huge disparity in vaccination coverage, we are simply not going to be able to achieve that."