Like Pfizer, Moderna is pressing the case for booster shots, although public health officials and experts have not yet agreed on when an additional dose would be needed and for which groups of people.

The company’s view is based on clinical trial data that show waning antibody levels against the coronavirus. Over time, Moderna expects that trend will “eventually impact” vaccine efficacy, although protection remains high through at least six months.

The final Phase 3 trial results showed 93% efficacy against symptomatic COVID-19, a number that hardly varied when analyzed by two-month intervals following full vaccination with two doses. Four months or more later, Moderna said, efficacy was 92.4%.

But the cut-off for collection of that data was March 26, before the delta variant had spread widely in the U.S. Data on neutralizing antibodies following vaccination, also shared by Moderna Thursday, show levels against the delta variant had dropped substantially by six months.

“We believe a booster is likely to be necessary this fall, particularly in the face of the delta variant,” said Moderna president Stephen Hoge on a Thursday conference call.

Moderna reiterated its belief that its vaccine would still protect against delta, but argued a third dose could boost antibody levels significantly higher. The company shared unpublished study results on 344 clinical trial participants who received a half-dose booster shot more than six months after their first two doses. Neutralizing antibody levels in that group rose above what was observed in Phase 3 testing, Moderna said.

“In our mind, the right way to benchmark this is ‘let’s look at that really high consistent efficacy in Phase 3 … and let’s do better, let’s exceed those titers,” said Hoge. “If we can, it should stand to reason we can provide protection at or above the levels we saw before.”

Moderna will wait to seek booster authorization, however, until it obtains data on a third shot given at the same dose level as what was authorized last fall. That data should be available in the coming weeks.

The company also expects to finish filing this month its application to the Food and Drug Administration for full approval of its vaccine, rather than the current emergency authorization. An approval is viewed by many experts as important for convincing people who remain skeptical of the vaccine.

Governments around the world are pressing ahead with booster dose preparations. France, Germany and Israel, for instance, are planning to offer additional doses despite growing concerns over glaring gaps in vaccine distribution and access.

Such steps would benefit Moderna, which said Thursday it has already signed advance purchase agreements or option deals with many high income countries for delivery of vaccine doses in 2022 and 2023.

Purchase deals already in place will translate to vaccine sales of $20 billion this year, Moderna said, and at least $12 billion in 2022.