CHICAGO (AP) — A false report claiming five Ukrainians had died after taking an American-made coronavirus vaccine spread in just a matter of days from a small Kremlin-friendly website to an audience of thousands in U.S.-based Facebook groups.

Russian media outlets picked up the claim, and soon social media users in the U.S. were sharing screenshots and links to those articles — all as 30,000 Americans were preparing to roll up their sleeves for shots of an experimental COVID-19 vaccine late last month.

The fast dissemination of a single report from an obscure Ukrainian website to crowds of Facebook users highlights the ease with which pro-Russian websites can feed misinformation into American internet circles. In fact, one of the websites that picked up the report was identified by the U.S. State Department this week as being part of a network of proxy misinformation websites being used by the Russian government.

As various countries race to produce a successful coronavirus vaccine, disinformation experts are bracing for a steady drum of misleading claims and propaganda aimed at undermining competing countries’ efforts to develop an antidote. Misinformation could raise distrust and fear around a vaccine, threatening government leaders’ hopes of ending the pandemic. And the U.S., which is readying plans to deliver 300 million doses to Americans starting next year, if a successful vaccine is identified, could be a prime target.

The falsehood around the U.S. vaccine fits into a longstanding, pro-Kremlin misinformation strategy, said Bret Schafer, a disinformation fellow at the Alliance for Securing Democracy.

“Anytime it appears the West has taken the lead in something they’re going to try to do damage,” Schafer said. “As they’re close to coming to market with their own vaccine, or start running trials, you create doubt about what everyone else is doing in the hopes that your own citizens look to you for the answers.”

Russia is boasting that it will soon become the first country to approve a COVID-19 vaccine. But scientists are raising serious doubts the country’s plans to start mass vaccinations as soon as October.

The unsupported claim that five out of 15 Ukrainians had died during a trial of the U.S. coronavirus vaccine was first published in July on the Lugansk Media Centre, a website affiliated with the separatist rebels in Ukraine’s Luhansk region, who are backed by Russia. The false report said eight others also were hospitalized. The vaccine, however, is not currently being tested in Ukraine, according to a global database of vaccine trials kept by both the World Health Organization and U.S. National Institutes of Health.

Lugansk Media Centre did not return a request for comment.

The report was picked up by Russian-friendly site NewsFront and was shared more than 20,000 times in public Facebook groups and pages, including several with large anti-vaccination followings in the U.S.

NewsFront was identified as a “proxy website” for the Russian government in a State Department’s report this week that detailed Russia’s apparatus of state-funded media, social media accounts and English-language websites used to spread disinformation.

“Vaccines are never safe!” one Facebook user wrote, sharing the NewsFront article in a Florida-based Facebook group called Vaxxed Vaccine Information.

The claim also gained traction in U.S. and French-speaking QAnon Facebook groups, which promote the conspiracy theory that President Donald Trump is waging a secret campaign against enemies in the “deep state” and a child sex trafficking ring.

And it appears to have inspired a new wave of memes aimed at sowing doubt about the vaccine.