Moderna says its COVID-19 vaccine is about 93 per cent effective through to six months after the second dose, showing hardly any change from the 94 per cent efficacy reported in its original clinical trial.

However, the company said it still expects booster shots to be necessary ahead of the winter season as antibody levels are expected to wane.

Moderna and Pfizer and partner BioNTech SE , which make a similar messenger RNA-based vaccine, have been advocating for a third shot to maintain a high level of protection against COVID-19, especially as more contagious variants of the virus spread widely.

Moderna Chief Executive Stephane Bancel said during a conference call that the company would not be able to produce more than the 800 million to 1 billion doses of the vaccine it has already targeted for this year.

“We are not taking any more orders for 2021 because we are we are totally maxed out,” he said.

The new Moderna data comes from the company’s roughly 30,000-participant clinical trial used to support the vaccine’s emergency authorisation in December.

It compares favourably to data from Pfizer and BioNTech last week that showed their vaccine’s efficacy waned about 6 per cent every two months, declining to around 84 per cent six months after the second shot.

While the shots are similar, they are not identical. Moderna’s doses contain 100 micrograms of vaccine, while Pfizer’s contain 30 micrograms.

“Our COVID-19 vaccine is showing durable efficacy of 93 per cent through six months, but recognise that the Delta variant is a significant new threat so we must remain vigilant,” Bancel said.