Moderna’s coronavirus vaccine booster appears to produce a “robust” antibody response against the fast-spreading delta variant, the company said Thursday as it warned that a third shot would “likely” be needed this fall.

The Massachusetts-based drugmaker revealed in a quarterly-earnings report that its original two-dose vaccine regimen remains highly effective through six months after the second shot, but the company believes that the “increased force of infection resulting from delta” will lead to a surge of breakthrough infections in vaccinated people over the next few months.

The delta variant is said to be as contagious as chickenpox and has already become the dominant strain in the U.S. and many other countries.

“We are pleased that our COVID-19 vaccine is showing durable efficacy of 93 percent through six months, but recognize that the Delta variant is a significant new threat so we must remain vigilant,” Moderna CEO Stephane Bancel said in a statement.

Moderna’s 93 percent efficacy rate after six months is higher than Pfizer’s estimate for its own vaccine, which the New York company reported last week to be about 84 percent, and it’s just slightly lower than the 94.1 percent rate in the immediate days after full immunization.

“While we see durable Phase 3 efficacy through 6 months, we expect neutralizing titers will continue to wane and eventually impact vaccine efficacy,” Moderna said in a slide presentation shared online. “Given this intersection, we believe dose 3 booster will likely be necessary prior to the winter season.”

It’s unclear when the drugmaker will complete its booster-vaccine study. The Phase II trials involve three booster candidates, all of which appear to offer strong protection against delta and other variants, including gamma and beta, according to Moderna.

The study is testing a single 50-microgram dose in participants who have previously received both of their 100-microgram vaccine shots, the company said.

Moderna also announced Thursday that it plans to seek full approval for its regular two-dose vaccine regimen this month. The company has been distributing the shots under an emergency-use authorization. It has also received provisional or conditional authorization from more than 50 other countries, according to a news release.

Plans for a third round of vaccination has infuriated global health experts worried about vaccine inequity. The World Health Organization on Wednesday called for a moratorium on vaccine boosters until at least 10 percent of the population of every country is vaccinated.

“To make that happen, we need everyone’s cooperation, especially the handful of countries and companies that control the global supply of vaccines,” WHO Director-General Tedros Adhanom Ghebreyesus said at a news conference.

France, Germany and the U.K. are among the countries already preparing for a booster vaccine rollout targeting vulnerable populations.