The footage is concerning, even alarming. In a number of videos posted online and already viewed millions of times on social media, two separate American women appear to experience bouts of tremors and involuntary body movements.

There is no direct connection between the women, except both had received COVID-19 vaccines shortly before their symptoms appeared, a fact some have linked to the appearance of these tremors. But scientists say there is another valid explanation for what we see in the footage.

While there's absolutely no suggestion that anyone is faking these symptoms, researchers think it's not the contents of the COVID-19 vaccines causing the shaking and convulsions.

Instead, it's possible that these viral videos depict a rare and little-understood medical condition called functional neurological disorder (FND) – a neuropsychiatric disorder thought to be triggered by a range of stimuli, including physical or emotional events, injuries, medical procedures, and sometimes even the act of getting a needle injected.

"Some people with FND have a heightened awareness of their body and increased state of arousal and threat, which may hijack normal neural networks controlling voluntary movements," says neurologist David Perez from Massachusetts General Hospital (MGH).

"FND teaches us quite a bit about the complexities of the human brain."

Functional neurological disorder, sometimes also known as conversion disorder, can't definitively be diagnosed from watching videos, but nonetheless the kinds of body control issues seen – limb weakness, gait problems, jerky movements, tremor, and facial spasms – are all symptoms of FND.

For that reason, the US-based FND Society issued a press release in January shortly after the videos began garnering attention on social media, observing that the clinical features of FND were a match for what appeared in the footage.

"We would expect FND to develop in some individuals after vaccination due to a combination of heightened stress owing to the pandemic, feelings of uncertainty about the vaccine and the normal transient physical symptoms, and discomfort after vaccination," the FND Society wrote.

Now, to make a similar kind of point and to help boost public understanding of FND, Perez and his co-authors have penned a new commentary in JAMA Neurology, observing that the videos do look like potential episodes of FND, which – if true – could have been triggered by the vaccine injections, but probably not by the vaccines themselves.

"Precipitating factors, while proximal to the development of the symptoms, are not directly caused by the substances in the vaccine in the same manner that, for example, Neisseria meningitidis is the cause of meningitis," the researchers explain.

"Instead, factors such as expectations, beliefs, heightened bodily attention, arousal, and threat/emotional processing play important mechanistic roles in the pathophysiology of FND."

It's a particularly important theme right now, the researchers urge, because videos like this sometimes attract thousands of views, and may also be shared by those who advocate for conspiracy theories and anti-vaccination beliefs.

In the face of a modern health crisis like COVID-19, it's important to get the word out on the strong possibility that these videos actually show rare instances of FND.

"The spread of these videos could fuel vaccine hesitancy by giving an overly simplistic impression of potential links between the vaccine and major neurological symptoms," says Perez.

"Instead, these are symptoms of a real, brain-based disorder that sits at the intersection of neurology and psychiatry."

So far, US federal health authorities such as the CDC haven't had much to say about FND's potential role in videos like this, mainly emphasizing that side effects to COVID-19 vaccinations tend to be "mild and moderate and go away quickly".

That's an important clarification the public needs to hear, but it doesn't go far enough in terms of educating people on what FND really is, the researchers say, especially when viral videos of scary-looking shaking symptoms are scoring millions of views – uploaded by people who are claiming the vaccines are directly responsible.

"A lack of direct messaging may be falsely perceived by the public that the Centers for Disease Control and Prevention is not properly surveilling adverse symptoms or, even worse, concealing them," the researchers explain.

"We must explain transparently and non-judgmentally the nature of FND, including that these symptoms are real but not the direct result of toxic vaccine effects."

The findings are reported in JAMA Neurology.