An 'effective' Covid-19 vaccine will still not stop people from getting sick and dying, scientists have warned.

They say trials being carried out by the world's leading pharmaceutical companies are not powerful enough to guarantee the jab will save lives.

Several vaccines have entered the most advanced stage of testing - known as 'phase three' - and could be declared effective in the near future.

But the trials are only investigating mild cases and not whether it will help the most vulnerable, according to a report by the British Medical Journal (BMJ).

BMJ associate editor Dr Peter Doshi said: "None of the trials currently underway are designed to detect a reduction in any serious outcome such as hospitalisations, intensive care use, or deaths.

"Nor are the vaccines being studied to determine whether they can interrupt transmission of the virus."

Coronavirus vaccine

The vaccines are being trialled by some of the world's leading pharmaceutical companies, including Pfizer, Moderna and Johnson & Johnson.

People taking part in the trials only have mild symptoms, meaning they have tested positive for Covid-19 and have a cough, the researchers found.

Dr Doshi said: "Part of the reason may be numbers. Because most people with symptomatic Covid-19 infections experience only mild symptoms, even trials involving 30,000 or more patients would turn up relatively few cases of severe disease."

Vaccine manufacturers have done little to "dispel" public opinion that an effective vaccine would put an end to the deadly virus, the researchers say.

Moderna, for example, has described hospitalisation as a "key secondary endpoint", but its trial does not have the statistical power to assess it, the company's chief medical officer Dr Tal Zacks told the BMJ.

Dr Doshi said: "Hospitalisations and deaths from Covid-19 are simply too uncommon in the population being studied for an effective vaccine to demonstrate statistically significant differences in a trial of 30,000 people.

Doctor disinfects skin of patient before vaccination

"The same is true regarding whether it can save lives or prevent transmission: the trials are not designed to find out."

The size and duration of the trial would need to be "vastly" increased to prove the vaccine would actually prevent hospitalisation, Dr Zacks confirmed.

Dr Zacks, speaking to The BMJ, said: "Neither of these I think are acceptable in the current public need for knowing expeditiously that a vaccine works.

"Moderna's trial is designed to find out if the vaccine can prevent Covid-19 disease. Influenza vaccines protect against severe disease better than mild disease."

A vaccine would be hailed effective if it reduced the risk of developing lab-confirmed Covid-19 among 30 per cent of participants, which is consistent with FDA and international guidelines.

Dr Doshi said: "To Moderna, it's the same for Covid-19: if their vaccine is shown to reduce symptomatic Covid-19, they will feel confident it also protects against serious outcomes."

Despite their obvious vulnerability, few trials have been designed to find out whether the vaccine benefits elderly people, the researchers also found.

Dr Doshi said: "If the frail elderly are not enrolled into vaccine trials in sufficient numbers to determine whether there is a reduction in cases in this population, there can be little basis for assuming any benefit against hospitalisation or mortality."

There is still time to make sure trials address these issues, according to the researchers.

Dr Doshi added: "The covid-19 vaccine trials may not have been designed with our input, but it is not too late to have our say and adjust their course. With stakes this high, we need all eyes on deck."

Final results will be published once about 150 patients develop symptoms.