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.

Moderna shares fell 3.6% to around $403.87 in pre-market trading after closing at $419.05 on Wednesday.

The Moderna data compares favourably to that released by Pfizer and BioNTech last week in which they said their vaccine's efficacy waned around 6% every two months, declining to around 84% six months after the second shot.

Both the Moderna and Pfizer-BioNTech vaccines are based on messenger RNA (mRNA) technology.

"Our Covid-19 vaccine is showing durable efficacy of 93% through six months, but recognize that the Delta variant is a significant new threat so we must remain vigilant," Bancel said.

The comment comes as public health officials across the world debate whether additional doses are safe, effective and necessary even as they grapple with the fast-spreading Delta variant of the coronavirus.

Meanwhile, Pfizer is planning to seek authorization for a third shot later this month, and some countries like Israel have begun or plan to start administering a booster shot to older or vulnerable people.

BOOSTER CANDIDATES

Separately, Moderna said its studies of three different booster candidates induced robust antibody responses against variants, including the Gamma, Beta and Delta variants.

It said neutralizing antibody levels following the boost approached those observed after the second shot.

For this year, Moderna has signed vaccine contracts worth $20 billion in sales. It has agreements for $12 billion in 2022, with options for another roughly $8 billion in sales and expects to produce between 2 billion and 3 billion doses next year.

The company, however, has not been able to keep pace with the much larger Pfizer, which expects to manufacture as many as 3 billion doses this year and 2021 sales to top $33.5 billion.

Moderna's vaccine was authorized for emergency use in adults in the United States in December and has since been cleared for emergency or conditional use in adults in more than 50 countries.

The company expects to finish its submission for full approval with the U.S. Food and Drug Administration this month.

It posted second-quarter sales of $4.4 billion, slightly above expectations of $4.2 billion drawn from 10 analysts polled by Refinitiv. Its Covid-19 shot is the firm's first authorized product and sales were just $67 million a year earlier.

Moderna earned $2.78 billion, or $6.46 a share, beating quarterly expectations of $5.96 a share.