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Bill Gates says it will likely take multiple doses of his coronavirus vaccine to protect against the virus.

Gate now says you will need to purchase several doses of his vaccine -- and even then he's not promising you will be protected.

In an interview with CBS Evening News' Norah O'Donnell, Microsoft co-founder and billionaire philanthropist Bill Gates said that it is likely that an effective coronavirus vaccine will require multiple doses.

"None of the vaccines at this point appear like they'll work with a single dose," Gates said. "That was the hope at the very beginning. Maybe one of them particularly in the second generation will surprise us. We hope just two, although in the elderly sometimes it takes more, and so making sure we have lots of elderly people in the trial will give us that data."

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Gates, who expects to earn billions from the sale of his vaccines, says there needs to be 70 to 80 percent coverage globally for herd immunity to occur.

In other words, most people in the world (at least 6 billion) will need to take multiple doses of the Gates' vaccines -- which isn't cheap.

This is bad news for the most vulnerable populations: the elderly, HIV-positive people, smokers, and people with preexisting conditions.

Experts predicted a working vaccine for COVID-19 would be difficult to develop due to the various strains of coronavirus. A coronavirus causes the common cold but, there are no vaccines for colds.

Normally it takes several years of animal and human clinical trials before a vaccine is brought to market. But the COVID-19 vaccine is being fast-tracked to hit the market within months - despite the low death toll.