The vast majority of advertisements on Facebook spreading misinformation about vaccines are paid by just two organisations, according to a study that highlights the role of social media in giving a platform to unscientific anti-vaccine messages.

Researchers, including those from the University of Maryland in the US, found that a small group of anti-vaccine ad buyers has successfully leveraged Facebook to reach targeted audiences.

The study, published in the journal Vaccine, also showed that the social media platform's efforts to improve transparency have actually led to the removal of ads promoting vaccination and communicating scientific findings.

The research calls attention to the threat of social media misinformation as it may contribute to increasing 'vaccine hesitancy,' which the World Health Organization (WHO) ranks among the top threats to global health this year, the researchers said.

This increasing reluctance or refusal to vaccinate threatens to reverse the progress made in halting vaccine-preventable diseases, such as measles, which has seen a 30 per cent increase in cases globally, they said.

The researchers examined more than 500 vaccine-related ads served to Facebook users and archived in Facebook's Ad Library.

This archive, which became available in late 2018, catalogued ad content related to 'issues of national importance.'

The findings show that the majority of advertisements (54 per cent) which opposed vaccination, were posted by only two groups funded by private individuals, and emphasised the purported harms of vaccination.

'The average person might think that this anti-vaccine movement is a grassroots effort led by parents, but what we see on Facebook is that there are a handful of well-connected, powerful people who are responsible for the majority of advertisements. These buyers are more organised than people think,' said Amelia Jamison, a faculty research assistant at the University of Maryland.

In contrast, those ads promoting vaccination did not reflect a common or organised theme or funder, and were focused on trying to get people vaccinated against a specific disease in a targeted population, the researchers said.

As Facebook categorises ads about vaccines as 'political,' it has led the platform to reject some pro-vaccine messages, they said.

'By accepting the framing of vaccine opponents -- that vaccination is a political topic, rather than one on which there is widespread public agreement and scientific consensus -- Facebook perpetuates the false idea that there is even a debate to be had,' said David Broniatowski, associate professor at the George Washington University in the US.

'This leads to increased vaccine hesitancy, and ultimately, more epidemics. Worse, these policies actually penalise pro-vaccine content since Facebook requires disclosure of funding sources for 'political' ads, but vaccine proponents rarely think of themselves as political,' said Broniatowski.

He noted that vaccine opponents are more organised and more able to make sure that their ads meet these requirements.

Facebook is a pervasive presence in the lives of many people, meaning its decisions about how to handle vaccine messaging have far-reaching and serious consequences, said Sandra Crouse Quinn, a professor at the University of Maryland.

'In today's social media world, Facebook looms large as a source of information for many, yet their policies have made it more difficult for users to discern what is legitimate, credible vaccine information. This puts public health officials, with limited staff resources for social media campaigns, at a true disadvantage, just when we need to communicate the urgency of vaccines as a means to protect our children and our families,' said Quinn.

The researchers note that the data gathered for this study from Facebook's Ad Archive was collected in December 2018 and February 2019, before the social media network's March 2019 announcement of updated advertising policies designed to limit the spread of vaccine-related misinformation.