Moderna’s coronavirus vaccine remains 93% effective six months after the second dose, the company said on Thursday, not much change from the 94% efficacy reported in November after its original clinical trial.

“We are pleased that our Covid-19 vaccine is showing durable efficacy of 93% through six months, but recognise that the Delta variant is a significant new threat so we must remain vigilant,” Stéphane Bancel, the chief executive officer of Moderna, said in a statement.

Last week, Pfizer had said its vaccine’s efficacy declined around 6% every two months, dropping to around 84% six months after the second shot.

Moderna, meanwhile, reiterated its belief that a third dose of the Covid-19 vaccine as a booster shot will be necessary later this year as antibody levels are expected to wane.

It said Phase 2 studies showed that its booster candidate induced a “robust antibody response” against Covid variants of concern, including the Delta strain which has spread to 135 countries since it was first identified in India.

Germany, France and Israel have decided to administer boosters to those potentially vulnerable to the virus. However, the World Health Organization on Wednesday called for a pause in administering booster shots, at least till September, so that supplies can be used to help all countries vaccinate at least 10% of their population. Even health experts and scientists have not arrived at a consensus on whether additional doses are needed to boost immunity in fully vaccinated people.