The arrival of the omicron variant has triggered a global rush for booster shots, as scientists and governments see a third dose as the most expedient strategy against the new strain that appears to cause a marked loss of vaccine protection.

In the days after earliest findings showed that omicron caused a 25 to 40-fold loss of neutralizing antibodies from the Pfizer-BioNTech vaccine, the second-most used shot across the world, the U.S. expanded booster access to teenagers while countries like the U.K. and South Korea are slashing wait times for a third dose in half, to three months.

More governments are certain to follow, while places less awash with vaccines in the developing world are trying to secure additional stock from manufacturers. BioNTech founder Ugur Sahin said earlier this week that the mRNA shot “should be a three-dose vaccine” to fight the new strain, and the third dose could come as early as three months after the second.

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But questions remain over whether the rush to booster is the right strategy against omicron.

The World Health Organization has expressed reservation, emphasizing that the world must work to ensure vaccine access to those yet to receive their first doses before richer governments roll out boosters to the general population. That’s the only way to prevent the emergence of new strains like omicron, it said. News two omicron cases in Singapore had already received third shots coursed through financial markets, damping bullishness fueled by the Pfizer-BioNTech studies earlier in the week.

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Researchers also caution that the science has yet to definitively show that shorter intervals for boosters than the current six-month norm creates higher immune protection against omicron or other variants.

“There is no data yet on whether they are needed three or six months after, and there could be a difference,” said Jin Dong-Yan, a virologist at Hong Kong University.

Complicating matters is emerging evidence that boosters given later rather than sooner could be more potent when it comes to inactivated vaccines like the one made by China’s Sinovac Biotech. While less effective against the original coronavirus and the delta strain than messenger RNA shots, Sinovac’s vaccine is the most widely used in the world — 2.3 billion doses have been shipped out, mostly to developing countries.

A study published in the Lancet medical journal this week found the increase in protective antibodies from Sinovac’s vaccine was significantly muted for those getting a third dose as soon as two months after the second, compared to those getting it eight months afterward.

Despite the lingering uncertainty, the gap between the unvaccinated and the boostered is already widening across the world and within individual countries.

In the U.S., fully vaccinated people are lining up for third shots amid concern over omicron, while the U.S. Centers for Disease Control and Prevention extended access to 16 and 17 year-olds this week. That’s driving a divide between those who are triple vaccinated and those who haven’t received any shots at all.

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On Friday, South Korea revised down the interval for booster shots to three months as one of Asia’s most-vaccinated countries grappled with a double whammy of record infections and the detection of omicron among patients.

U.K. Prime Minister Boris Johnson also promised to slash the six-month wait time between second and third doses by half next week amid signs omicron is spreading among Britons.

Elsewhere, especially among developing countries still struggling to procure adequate vaccine stock, anxiety is rising over booster access: In Vietnam, which is still battling a protracted virus wave that has disrupted its export-reliant economy, the government announced this week a target to give boosters to the entire population in the first six months next year, though less than 60% of its population are currently fully vaccinated.

In India, which has not yet started a third-dose campaign with only 36% of its population having received two doses, medical associations are calling for boosters for front-line health workers to keep them safe. South Africa, where omicron was first detected, said it will start rolling out boosters from early next month.

Apart from Pfizer and BioNTech, no other vaccine maker has yet released findings on how their shots hold up against omicron, and the implications of a shorter booster interval strategy. Little is known on how viral vector vaccines developed by companies including AstraZeneca Plc and Johnson & Johnson and inactivated shots made by the Chinese companies react to the new strain, compared to messenger RNA doses.

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Yet given the fear that omicron may reverse the world’s efforts to emerge from the pandemic, the booster race is likely only to accelerate in the coming weeks.

“Fully vaccinated will mean three doses,” said HKU’s Jin. “That will likely become the universal requirement across governments around the world.”