Anti-vaccine campaigners once confined to relatively obscure groups have exploited the coronavirus pandemic to reach a wider audience online, feeding on public fears to sow doubt about the drugs now available.
But while the "anti-vax" camp has long understood the importance of the information battle, says science historian Laurent-Henri Vignaud, the health authorities are often a step behind.
The problem, says Vignaud, co-author of "Antivax", a 2019 book on the movement, is that health officials are "starting from the principle that vaccination is useful" to the population -- the very premise attacked by antivaxers.
The modern antivax movement took off on the back of a long-discredited medical 1998 study published in The Lancet suggesting a link between the MMR vaccine (measles, mumps and rubella) and autism.
It got a foothold in special-interest groups such as some religious communities and fringe environmental campaigners. Then, over the last year, interest in their theories exploded.
Facebook groups peddling false information on the vaccines have attracted masses of followers, according to a BBC study published at the end of March, which studied Brazil, France, India, Kenya, Mexico, Tanzania and Ukraine.
In France, for example, pages sharing anti-vaccine content received nearly four million likes -- up 27 per cent. (That is a rate of growth three times faster than in 2019, but comparable to 2018.)
Antivax theories then are no longer limited to a handful of fringe groups.
You can find them online among France's yellow vest campaigners, among libertarian groups and New Age proselytisers, according to First Draft, a campaign group that specialises in exposing misinformation.
The movement has created strange bedfellows, including a variety of conspiracy theorists who have incorporated the antivax narrative into their world view to stay topical, says First Draft researcher Seb Cubbon. And their message appeals to both far-left and far-right groups, says sociologist Florian Cafiero of France's CNRS.
But in a 2020 study First Draft warned that "increasing rates of vaccine scepticism may not only jeopardise the effectiveness of a potential Covid-19 vaccine, but that of vaccines more broadly, and even levels of trust in institutions connected to science and medicine".
A handful of high-profile campaigners have pushed the antivax message online.
Researchers at the University of Zurich studying thousands of English-language tweets found that while the anti-vax message was being pushed by a small fraction of Twitter users, they were boosted by a strong level of interaction.
Some 65 per cent of online antivax content in February and March could be attributed to 12 "extremely influential creators", said the US-based Center for Countering Digital Hate. One of them is the lawyer Robert F. Kennedy Jr, nephew of the former president.
Millions of people around the world have been forced to submit to lockdowns at some point in the past year, and many of them have gone online looking for answers to the crisis that has disrupted so many lives.
But the initial lack of hard facts about the new threat, coupled with failures of communication by some official channels -- such as mixed messages about the effectiveness of masks -- was fertile ground for antivaxers.
And the fact that many ordinary people do not have scientific training made them vulnerable to their disinformation.
Even scientific successes, such as the swift development of vaccines using innovative methods, became a source of suspicion for the sceptics.
Once health workers started recording stronger-than-expected side effects for the AstraZeneca and Johnson & Johnson jabs, that only fed into conspiracy theory narratives.
The online misinformation -- sometimes in well-produced packages such as the documentary "Hold-Up" in France which alleged a "global manipulation" over the pandemic -- garnered millions of views.
Its allegations have been picked up and amplified by politicians, celebrities and online influencers.
Claims have spread online by anonymous "doctors" that the vaccines are ineffective or even in some cases deadly. Fake videos have appeared purporting to show people who died after being injected with a dose of vaccine.
Many of these claims have been examined and debunked by AFP's fact-checking team. AFP has written 700 articles fact-checking the claims about vaccines.
The major online players -- Facebook, Twitter and YouTube -- have stepped up efforts to track down and remove disinformation on their platforms, while promoting information from the health authorities.
But the antivax message is still all over the internet.
In September, the World Health Organization and several UN organisations expressed concern about the flood of misinformation about the pandemic, now referred to as an "infodemic".
"An infodemic is about much more than misinformation or disinformation," said Christine Czerniak, leading the WHO's fight against pandemic misinformation.
"It's also about an overwhelming amount of information -- and information gaps and confusing messaging -- that all together make it difficult for everyone to know what to do."
And the effects of such misinformation can be tragic, when people die trying fake cures, such as drinking bleach, she added.
Exposure to false or misleading information about vaccines tends to lower people's readiness to take them, according to a study by the Vaccine Confidence Project published in the March edition of the scientific review Nature.
"The public's willingness to accept a vaccine is therefore not static," it concluded.
"It is highly responsive to current information and sentiment around a Covid-19 vaccine, as well as the state of the epidemic and perceived risk of contracting the disease."
And the stakes are high, says Alain Fischer, president of body overseeing France's vaccine strategy.
"If too many people don't vaccinate, we'll never get to the vaccinated cover needed to acquire herd immunity," he says.
And that is what is required if the authorities are to lift the social distancing measures and other restrictions that have been in place for the better part of a year now, he adds.