Moderna could have to wait until as late as december to analyse data from its covid-19 vaccine trial, longer than would be necessary to meet the trump administrations hopes of issuing an emergency approval ahead of the us election.

The boston-based biotech company on thursday said it had enrolled more than 25,000 participants in its trial, with more than 10,000 of those having received both doses in the vaccine course.

It also set out a slower analysis timeline than rival pfizer and its partner biontech, which said they would have their key data by the end of october. shares in moderna fell 2.4 per cent to $67.22 on the news.

Stphane bancel, moderna chief executive, told cnbc that if the infection rate in the us slowed in the coming weeks, it may not be able to examine data until december.

But he added that the most likely scenario was an interim analysis in november and if infection rates increased, it could be earlier. our best plan is october. i think its unlikely but it is possible.

The trump administration is pushing back against public health experts estimates of when a vaccine may be widely available. robert redfield, director of the centers for disease control and prevention, on tuesday said that this would not happen until late 2021. president donald trump accused dr redfield of making a mistake and insisted a vaccine could be not only approved but also distributed in october.

Speaking at modernas investor day, mr bancel said a successful covid-19 vaccine could lead to a multibillion-dollar revenue line in 2021.

He added it could also validate the companys entire portfolio of potential vaccines, which includes candidates for rare diseases, the zika virus, and influenza, because they are all based on the same technology, the molecule messenger ribonucleic acid, which carries genetic instructions.

That is the power of an information molecule, he said, adding it could be a unique and historic inflection point for the company, which does not yet have any products on the market.

The speed of vaccine trials depends on rates of infection. to prove the vaccine is effective, trials must show that more people in the placebo group, who do not receive the real vaccine, became infected.the trial structure also means that moderna does not know how many participants have become infected with covid-19 so far.

The interim analysis dates are estimates based on when the trial started in july, how quickly participants were enrolled in the early weeks, and infection rates in the regions where the vaccine has been given, the company said.

It will not start to assess the data until 53 participants develop the disease, with a second analysis due when 106 people become symptomatic. the final analysis cannot happen until at least 151 participants are ill, which they estimate may not be until may next year. but a vaccine may receive emergency approval based on an interim analysis.

Moderna also published its protocol for the phase 3 stage of its vaccine trial that is now under way for the first time.