IT was the news everyone had longed for, but there had never been any guarantee would come: a vaccine that prevents Covid-19.

On Monday, Sir John Bell, an immunologist and regius chair of medicine at Oxford University, made headlines when asked in an interview on Radio 4 whether this meant “life should be returning to normal by Spring”?

His response was what we all wanted to hear: “Yes, yes, yes.”

If we needed anything to motivate us through a long, dark winter of hospitality curfews, travel bans, and digital Christmases, then this was it.

It is difficult to overstate the significance of the breakthrough by Pfizer and its German partner BioNTech as so many viruses - from the original SARS in 2003 to Ebola and HIV - have eluded vaccine scientists.

Many virologists were anticipating an effectiveness somewhere in the range of 50-70 per cent, so preliminary results of 90% efficacy were mind-blowing.

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Before we get too carried away, however, there are still hurdles to be overcome and plenty of questions to answer.

Firstly, the trial is not yet completed. It only began on July 27 and as of November 8, 4583 participants in the Phase 3 clinical trial (there are 43,538 participants in total, mostly in the US and Germany) had yet to receive their second dose of vaccine - be it placebo or the genuine candidate.

To date, 94 Covid-19 cases have been detected and the threshold required is 164 - although given the US and Europe are now in the grip of a second wave, this is not expected to take too long to accrue.

The ‘90% effective’ measure means that the vaccine prevented symptoms of Covid-19 in 90% of volunteers who received the vaccine compared to the placebo.

Although detailed data is yet to be published, that suggests that well over 80 of the confirmed Covid cases were detected in the placebo group who received the dummy vaccine.

So far, no serious safety concerns have emerged but it is likely that, on a final analysis, effectiveness will be lower than 90% . Few vaccines - with exceptions of measles and chickenpox - are 90% effective.

However, anything above 50% (with no safety problems) is good enough for emergency approval by regulators. The flu vaccine is only around 50% effective each year, but still saves millions of lives worldwide.

All going well, Pfizer/BioNTech expect to be in a position to apply for emergency use approval around the third week in November and, if granted, to manufacture 50 million doses by the end of the year and 1.3 billion by the end of 2021.

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The UK has 40 million on order, and may be able to vaccinate thousands of priority individuals - namely elderly care home residents, care home workers, and frontline NHS workers - before Christmas.

As we enter 2021, other Covid vaccines (there are at least nine others currently in the crucial and final Phase 3 stage of clinical trial) could also be licensed, allowing governments to ramp up mass immunisations.

This is one of the reasons for Sir John Bell’s optimism: now that we know one vaccine works, the prospects for the others have also increased.

What we don’t know - and can’t know for sure until the vaccine has been used on millions on people for two or three years - is exactly how long immunity from infection lasts.

We know that Covid antibodies wane within months of recovery from SARS-CoV-2 infection, but it is unclear how long they last from vaccination and in any case, antibodies are only one element of the immune response: T cells are also important, as are and memory B cells which can ‘recall’ pathogens and regenerate antibodies in response to repeat infections.

It seems likely that booster vaccinations will be required, maybe - like the flu jag - at annual intervals for at-risk groups. But it is also possible that being vaccinated once could protect against more serious Covid disease in future.

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We also do not know whether the Pfizer vaccine prevents transmission as well as disease - or even whether it only prevents symptoms of infection, rather than infection itself.

The vaccine prevented “clinically symptomatic infection”: it is possible that some participants could have had asymptomatic Covid infections.

It is also unclear whether the vaccine can stop people - with or without symptoms of disease - from spreading the virus.

However, even if the vaccine is only “disease-modifying”, that still has substantial benefits for the NHS in terms of reducing serious illness, hospitalisations, and deaths.

A final point is that we do not yet know whether the vaccine is equally effective across all age groups and ethnicities. Pfizer’s press release states that “42% of global participants and 30% of U.S. participants have racially and ethnically diverse backgrounds”, but clinical trial participants tend to be younger.

Nonetheless, this week's news is a triumph for science and a beacon of hope that the threat of lockdowns could be behind us within six months.