The United States' oldest anti-vaccine advocacy group often emphasises that it is supported primarily by small donations and concerned parents, describing its founder as the leader of a "national, grass roots movement".
But over the past decade a single donor has contributed more than US$2.9 million ($4.4m) to the National Vaccine Information Centre, accounting for about 40 per cent of the organisation's funding, according to the most recent available tax records.
That donor, osteopathic physician Joseph Mercola, has amassed a fortune selling natural health products, court records show, including vitamin supplements, some of which he claims are alternatives to vaccines.
In recent years, the centre has been at the forefront of a movement that has led some parents to forgo or delay immunising their children against vaccine-preventable diseases such as measles.
Health officials say falling vaccination rates contributed to the infectious virus sickening more than 1200 people in the United States this year, the most in more than 25 years.
Measles outbreaks are surging worldwide, including in Samoa — where nearly 80 people have died since mid-October, largely young children and infants.
The Northern Virginia-based National Vaccine Information Centre lists Mercola.com as a partner on its homepage and links to the website, where readers can learn about and purchase Mercola's merchandise.
Last month, Mercola wrote on his website that measles "continues to be a Trojan Horse for increasing vaccine mandates." A recently removed page said "vitamin C supplementation is a viable option for measles prevention". Elsewhere on the site, a page about vitamin D includes the headline, "Avoid Flu Shots With the One Vitamin that Will Stop Flu in Its Tracks".
Mercola, whose claims about other products have drawn warnings from regulators, has also given at least US$4m to several groups that echo the anti-vaccine message. His net worth, derived largely from his network of private companies, has grown to "in excess of US$100 million", he said in a 2017 affidavit.
Mercola said in emails to the Washington Post that he contributes to the centre because he believes in its mission.
He said he offers "simple, inexpensive and safe alternatives to the conventional medical system, which is contributing to the premature death of millions and is causing needless pain and suffering in great part because multinational corporations want to increase their revenues".
He declined to be interviewed and did not respond to questions about how much profit his vitamin D and C supplements generate relative to the rest of his wide-ranging merchandise, which includes organic cotton underwear and pet food.
Supplements containing those vitamins are among Mercola's "top products", his website says. His media team said the claims on Mercola's website relate to vitamin D and vitamin C generally and "do not mention Dr Mercola's products whatsoever".
This month Samoan anti-vaccine activist Edwin Tamasese, who touted vitamins as an alternative to vaccination, was arrested for allegedly claiming on social media that measles vaccinations would result in mass deaths.
"The anti-vaxxers unfortunately have been slowing us down," government spokesman Afamasaga Rico Tupai told TVNZ after the arrest.
"We've had children who have passed away after coming to the hospital as a last resort and then we find out the anti-vaccine message has got to their families and that's why they've kept these kids at home."
Vaccination rates dived on the tiny Pacific island in the past year after two infants died within hours of receiving the measles, mumps and rubella vaccine — fatalities that were highlighted by anti-vaccination activists.
Two Samoan nurses were later convicted of negligence causing manslaughter for improperly mixing the vaccine with a muscle relaxant instead of water.
America's National Vaccine Information Centre was founded in 1982 by Barbara Loe Fisher, who has said her son was injured by a vaccine.
The group claimed credit this year for helping to defeat legislation in a dozen states that would have made it harder for parents to opt out of vaccinating their children.
Mercola claimed that vaccines have been associated with "deaths and permanent neurological complications", and he said vitamin D supplements were among "far more effective, less expensive and less risky alternatives".
Such claims are highly misleading, government health officials say.
For example, they say, while on very rare occasions people have developed the neurological disorder Guillain-Barre syndrome after receiving the flu shot, research suggests the disorder is more strongly associated with contracting the flu itself than with receiving the vaccine.
In addition, while some studies have suggested that vitamin D might help prevent the flu, others have found no such benefit, and the Centres for Disease Control and Prevention have said that the best way to protect against the infection is vaccination.