Elon Musk shared his personal controversial experience with the second COVID booster shot, explaining he ran into "major side effects."

In a series of tweets, the Twitter CEO said he "was dying" and that a cousin of his in "peak health" grappled with myocarditis, an inflammation of the heart which is listed as a possible side effect of some coronavirus vaccines. 

"I had major side effects from my second booster shot. Felt like I was dying for several days. Hopefully, no permanent damage, but I dunno," Musk said in a tweet.

He added: "And my cousin, who is young & in peak health, had a serious case of myocarditis. Had to go to the hospital."

In a third post, Musk explained that he was forced to get the second booster shot inorder to visit a Tesla location in Berlin, Germany.

"Was required to visit Tesla Giga Berlin. Not my choice," he said.

In another post, Musk said he had no issues with receiving the initial Johnson & Johnson vaccine or the first mRNA booster.

"I had OG C19 before vaccines came out and it was basically a mild cold. Then had J&J vaccine with no bad effects, except my arm hurt briefly," he tweeted.

The Tesla founder added: "First mRNA booster was ok, but the second one crushed me."

Musk's posts came after Rasmussen Reports tweeted 12 million people may have experienced "major side effects" after getting the vaccine.

According to the Center for Disease Control and Prevention (CDC) cases of myocarditis are a "rare risk" for those who receive mRNA COVID-19 vaccines.

"These rare cases of myocarditis or pericarditis have occurred most frequently in adolescent and young adult males, ages 16 years and older, within 7 days after receiving the second dose of an mRNA COVID-19 vaccine (Pfizer-BioNTech and Moderna). There has not been a similar reporting pattern observed after receipt of the Janssen COVID-19 Vaccine (Johnson & Johnson)," the CDC says on its website.