The World Health Organization on Wednesday supported coronavirus vaccine booster shots across the world only if its need was warranted by evidence and that it was administered on priority to the population with comorbidities and healthcare workers.

The statement was issued at a time when the Omicron variant of coronavirus has spread to 106 countries. WHO has called Omicron a “variant of concern” because of its increased transmissibility, infectivity, or resistance to vaccines.

“The objective of a booster dose is to restore vaccine effectiveness from that deemed no longer sufficient,” the global health body said in a statement.

WHO’s call for booster doses follows the Strategic Advisory Group of Experts on Immunisation statement on October 4 on how vaccine protection against coronavirus saw “a minimal to modest reduction” over six months after the second dose.

According to the health body, emerging data has shown a decline in vaccine effectiveness against coronavirus with time, with the protection significantly declining in older adults.

“In adults above 50 years, vaccine effectiveness against severe disease decreased by about 10%,” it said. “Vaccine effectiveness against symptomatic disease decreased by 32% for those above 50 years of age.”

In this light, the data on protection offered by booster shots from various countries demonstrate “an improvement in protection against infection, milder disease, severe disease and death”, the health body said.

WHO added that at least 126 countries have recommended booster or additional vaccination and more than 120 nations have been rolling out the shots. It added that the majority of these countries are classified as high-income or upper-middle-income. No low-income country has yet introduced a booster vaccination programme.

India is yet to release an official statement on the status of booster shots in the country. The Indian SARS-CoV-2 Genomics Consortium, responsible for monitoring genome sequencing of coronavirus cases, has urged the government to consider rolling out booster shots at the earliest.

WHO also noted its concerns about the inequitable distribution of vaccines across the world, with lower-income countries having less access. It added that many countries are yet to vaccinate 40% of their population – a threshold that is deemed important to prevent a wave of coronavirus from affecting a region at any point.

The health body said currently, 20% of the coronavirus vaccines were being used as a booster or additional doses across the world.

“In view of the continued supply uncertainties in global vaccine access and equity, individual country vaccine booster dose policy decisions need to balance the public health benefits to their population with support for global equity in vaccine access necessary to address the virus evolution and pandemic impact,” it said.