Moderna Inc (MRNA.O) said on Thursday its COVID-19 shot was about 93% effective through six months after the second dose, showing hardly any change from the 94% efficacy reported in its original clinical trial.

However, it said it still expects booster shots to be necessary ahead of the winter season as antibody levels are expected to wane. It and rival Pfizer Inc (PFE.N) and BioNTech SE (22UAy.DE) have been advocating a third shot to maintain a high level of protection against COVID-19.

During a second-quarter earnings call, Moderna CEO Stephane Bancel said that the company would not produce more than the 800 million to 1 billion doses of the vaccine that it has targeted this year.

“We are now capacity constrained for 2021, and we are not taking any more orders for 2021 delivery,” he said.