Bill Gates today warned anti-vaxxers could wreck attempts to develop a Covid-19 vaccine if they refuse to take it and reduce the level of herd immunity.
Over 80 per cent of people may need to have the jab for it to work properly - but he feared anti-vaccine 'craziness' might put people off getting it.
The billionaire founder of Microsoft, who now donates hundreds of millions of dollars to global health causes, said the prospect was 'worrying'.
Vaccines can only be successful at stamping out a virus if so many people get them that a vast majority of the population is immune and the disease can no longer spread.
Falling levels of the virus circulating in Britain, where the outbreak is fading, means it will be increasingly difficult to test the vaccine because there is nothing to test it against.
In Brazil, however, Covid-19 cases are still rising rapidly and its outbreak is second only to the US, with 555,000 confirmed diagnoses.
Mr Gates, whose foundation has donated $250million to the hunt for a coronavirus vaccine, said scientists would want more than 80 per cent of people to have the Covid-19 jab if one is developed
Scientists around the world are scrambling to try and develop a vaccine for the coronavirus. A jab is seen as the only way of stamping out the virus for good, in the same way it did for measles and polio (stock image)
The Bill and Melinda Gates Foundation, which Gates runs with his wife, has donated more than $250million (£200million) to the search for a coronavirus vaccine.
A vaccine is considered to be the only way to stamp out the virus because it is too deadly for countries to leave it to run unchecked.
The widespread use of a vaccine would build up herd immunity to virus, meaning that so many people are immune to it that it doesn't have a big enough pool of potential victims to cause a proper outbreak.
There will always be people who cannot have vaccines - those whose immune systems don't work, such as cancer patients, for example - so high uptake in the healthy population is crucial.
Experts say at least two-thirds of people would need to be immune to Covid-19 to stop another epidemic.
WHAT ARE BRITAIN'S BEST VACCINE HOPES?
Vaccines being developed by Oxford University and Imperial College London are currently Britain's best hopes.
Oxford's vaccine is already in safety trials in human patients around the UK, and Imperial intends to start soon.
The science behind both vaccine attempts hinges on recreating the 'spike' proteins that are found all over the outside of the COVID-19 viruses.
Both will attempt to recreate or mimic these spikes inside the body. The difference between the two is how they achieve this effect.
Imperial College London will try to deliver genetic material (RNA) from the coronavirus which programs cells inside the patient's body to recreate the spike proteins. It will transport the RNA inside liquid droplets injected into the bloodstream.
The team at the University of Oxford, on the other hand, will genetically engineer a virus to look like the coronavirus - to have the same spike proteins on the outside - but be unable to cause any infection inside a person.
This virus, weakened by genetic engineering, is a type of virus called an adenovirus, the same as those which cause common colds, that has been taken from chimpanzees.
If the vaccines can successfully mimic the spikes inside a person's bloodstream, and stimulate the immune system to create special antibodies to attack it, this could train the body to destroy the real coronavirus if they get infected with it in future.
The same process is thought to happen in people who catch COVID-19 for real, but this is far more dangerous - a vaccine will have the same end-point but without causing illness in the process.
The higher the uptake of a vaccine, the better it works. For measles, the target is to vaccinate 95 per cent of the population.
But anti-vaxxers refuse to have jabs or give them to their children because of conspiracy theories suggesting they are harmful or used only to make money.
Mr Gates said on Radio 4 this morning: 'It is troubling that in times like that, and accelerated by digital tools, there is so much craziness.
'Eventually when we have the vaccine, we will want to develop the herd immunity to have over 80 per cent of the population taken.
'If they have heard that it is a plot, or vaccines in general are bad, and we don't have people willing to take the vaccine, then that will let the disease continue to kill people.
'So it is a bit worrying that there is some of that crazy stuff.'
One bogus theory suggests Bill Gates, who made his fortune as a computer magnate, plans to use a vaccine to implant microchips in people, and that the global pandemic which has killed almost 400,000 people is a cover-up.
The theory is based on a suggestion by Mr Gates that there would one day be 'digital certificates' to show who had been vaccinated or had Covid-19 already, like medical records, the BBC reported.
The Foundation was forced to point out that claims it wanted to track people or use microchips were 'false'.
Mr Gates added on Radio 4: 'I'm kind of surprised that some of that is focused on me.
'We are just giving money away to get there to be a tool.
'We just write cheques to pharma companies [and] we happen to have a lot of the smart pharmaceutical expertise in our foundation, and are considered a fair broker between governments and the companies to help pick the best approach.'
One of the fastest moving candidates for a vaccine is that being developed by the University of Oxford, which is already in human trials in the UK.
The vaccine has recruited thousands of people to test the safety of the jab, but testing whether it actually works will become a tall order now that the virus is fading out in Britain.
As a result, the team are shipping it to Brazil, which is now in the grip of the world's fastest growing outbreak.
There have been more than half a million people diagnosed there and more than 30,000 have died.
The vaccine will be tested on 2,000 people working in healthcare environments between the ages of 18 and 55, said the Federal University of Sao Paulo, which is in charge of the study.
The president of the university, Soraya Smaili, said the volunteers 'must be health professionals between 18 and 55 years old and be at high risk of infection, for example, cleaning and support staff in units treating COVID-19 patients'.
Professor Smaili added the vaccine was being tested in Brazil 'ecause we are in the acceleration phase of the epidemiological curve'.
Britain, on the other hand, is coming out the other side of its peak and case numbers are declining, meaning it will be hard to measure the effects of a vaccine because so few people are getting infected.
New cases of coronavirus are soaring in Brazil, with tens of thousands of people being diagnosed with the virus every day there
The current best estimate is that around 8,000 people per day are catching the virus, with around 54,000 infected at any given time.
The university added: 'The results will be fundamental for the vaccine's approval in the United Kingdom, expected late this year'.
Pharmaceutical giant AstraZeneca has agreed to mass-produce Oxford's vaccine and help to distribute it if it becomes successful.