Questions remain about just how effective Pfizer's coronavirus vaccine really is, but the early signs are "a cause for real optimism", according to one of the world's top vaccine experts.
The New York-based pharma giant and its German partner BioNTech on Tuesday said its vaccine against the deadly disease appeared to be 90 percent effective, based on early data from a trial involving more than 40,000 people, with no serious safety concerns cropping up.
Pfizer chief executive Albert Bourla called it a "great day for science and humanity".
"We are reaching this critical milestone in our vaccine development program at a time when the world needs it most with infection rates setting new records, hospitals nearing over-capacity and economies struggling to reopen."
University of Oxford public health expert Peter Drobac told The AM Show on Tuesday it was "unequivocally exciting news, and I think a cause for real optimism".
"For the first time, a vaccine candidate can say that it actually protects people from getting sick with this virus. The 90 percent effectiveness number that was cited, if that holds up, it really exceeds most of our expectations and would be very, very promising."
The Pfizer/BioNTech vaccine, called BNT162b2, works by teaching the immune system to recognise the virus' spike protein - the pointy bit that you see in illustrations of the virus.
Like many others, it's now in phase three trials - when it's given to thousands of people, while thousands of others receive a placebo - if people in the latter group fall sick while people in the former don't, then the vaccine works. After one month of the phase three trial, there had been 94 cases of COVID-19 recorded amongst those taking part.
"The case split between vaccinated individuals and those who received the placebo indicates a vaccine efficacy rate above 90 percent, at seven days after the second dose," Pfizer and BioNTech said. "This means that protection is achieved 28 days after the initiation of the vaccination, which consists of a two-dose schedule."
"We don't yet know whether it prevented against severe infection, but it's quite exciting," said Dr Drobac.
"The US Federal Drug Administration set a threshold of 50 percent effectiveness for a vaccine to be good enough to get approved. No vaccine is really 100 percent effective - 90 percent is awfully good."
The two companies say this figure might change however once the trial is complete - they're waiting for a few dozen more infections amongst trial participants before doing a final analysis. And while the data collected to date was analysed by an independent group, it hasn't yet been submitted for peer review or published in a scientific journal.
Dr Drobac says the current data also only covers the first 28 days - it's not clear how long the vaccine might offer protection. Getting infected doesn't appear to offer long-lasting immunity, with reports of secondary infections sometimes causing symptoms more severe than the initial illness.
"It's going to take some time before we really understand how long a vaccine like this might be effective for."
New Zealand in October signed a deal with Pfizer and BioNTech to buy 1.5 million doses of the vaccine, to be delivered in 2021 - enough for 750,000 people. It's also bought into a World Health Organization-led vaccine effort, and set aside money for other potential candidates.
Pfizer and BioNTech have already put their vaccine into production, optimistic it'll get emergency approval from US authorities in the next few weeks.
Dr Drobac's Oxford is also developing a vaccine, with British-Swedish pharma company AstraZeneca. Its trial is expected to wrap up by the end of 2020, but - like Pfizer's vaccine - has already gone into production.
"Most of the lead vaccine candidates are doing the same thing - they're signing pre-purchase agreements with individual countries and also some international bodies as well," said Dr Drobac.
"If the vaccine was approved tomorrow, hey that's great - but you have to be able to manufacture hundreds of millions, if not billions of them, and distribute them around the world. The idea is let's bet on a couple of horses early on... so we've already got a couple of hundred million at the time that one of them, or hopefully more than one of them, are demonstrated successful."
First in line to get the vaccine would likely be healthcare workers, he said, followed by those working in aged care and people vulnerable to severe illness if they caught it.
"Whether it's the Pfizer vaccine or another, when they do ultimately get approval I would feel very confident they're both safe and effective and encourage folks not to be afraid to get one."
In response, Russian authorities said its Sputnik V vaccine was also 90 percent effective, but has yet to provide any phase three trial data.
"We are responsible for monitoring the effectiveness of the Sputnik V vaccine among citizens who have received it as part of the mass vaccination programme,' said Oksana Drapkina, director of a research institute under the Russian health ministry.
"Based on our observations, its effectiveness is also more than 90 percent. The appearance of another effective vaccine - this is good news for everyone."
Alexander Gintsburg, director of vaccine developer Gamaleya Institute, said their phase three trial data would be published soon.
"I am sure that its effectiveness level will also be high."