Elon Musk has cast doubt on the safety of the second COVID-19 jab in a tweet to his millions of followers

• Elon Musk baselessly said the second coronavirus jab could be more harmful.
• While a second dose of the Pfizer jab can cause stronger side effects, they only last a few days.
• Musk has previously downplayed the risk of the virus, which has killed more than 500,000 Americans.

Elon Musk baselessly told millions of his followers on Friday that there's been "some debate" about the second coronavirus injection being more harmful than the first, although experts have overwhelmingly said vaccines are safe and that side effects will subside within days.

The Tesla CEO was responding to a tweet written by author Ashlee Vance, who said his elderly parents were refusing to get the coronavirus vaccine because of the "stuff they saw on Facebook."

"For sure wise for elderly or immunocompromised to take the vaccine. Some debate about the second jab though. Quite a few negative reactions to that," Musk tweeted to his 48 million followers in response.

According to research by the Centers for Disease Control and Prevention, allergic reactions to Pfizer's and Moderna's first and second shots are extraordinarily rare in the US, with about 4.5 reactions per million doses - or 0.00045%.

Studies of Pfizer's coronavirus vaccine showed no major safety concerns.

Most of the stronger side effects of the Pfizer vaccines, which include a headache, fever, and fatigue, occurred after participants got the second dose, according to research data by the FDA.

But the numbers are still low. The FDA research showed 31 percent of people between the ages of 18 and 55 receiving the second dose of the Pfizer vaccine reported a fever. With the Moderna vaccine, 17 percent in the same age group reported getting a fever after the second dose.

The side effects were expected with the vaccine and typically only last for a day or two on average. Side effects are more common among younger participants.

Musk's tweet comes after The Washington Post reported Friday that more than 400 of the roughly 10,000 workers at Tesla's California factory tested positive for COVID-19 from May to December - a much bigger outbreak than previously thought. At the time, Musk had chosen to reopen the factory despite local state orders.

Musk has downplayed the risk of the virus, which has killed more than 500,000 Americans, on multiple occasions.

In March last year, the Tesla CEO tweeted the "coronavirus panic is dumb" and also predicted the US would be reporting "close to zero new cases" by the end of April. Musk also vouched for an approach that would allow mass infection before vaccines become available.