WASHINGTON -- People could need multiple vaccine doses to immunize themselves against the coronavirus and deploying a vaccine will require a global effort, Microsoft co-founder Bill Gates said on Wednesday.

"None of the vaccines at this point appear like they'll work with a single dose," Gates told CBS Evening News. "That was the hope at the very beginning."

He noted that the multiple doses could require more than 7 billion vaccinations to be administered worldwide if necessary.

Gates has been warning about the threat of a global pandemic since 2015, and has donated $300 million towards the global effort to combat COVID-19 through the Bill & Melinda Gates Foundation. He admitted that "there will be a lot of uncertainty" about the efficacy of any vaccine, but stressed that it's a solution "that will improve over time."

On Tuesday, US President Donald Trump resumed his press briefing on the coronavirus pandemic, admitting that the situation in the country will "get worse."

On development of vaccines and therapeutic cures, the president signaled willingness for international cooperation on vaccines, saying that "we're willing to work with anybody that's going to get us a good result" when asked if the administration would collaborate with China.

As of late Wednesday night, the number of confirmed COVID-19 cases in the United States neared 4 million with more than 143,000 deaths, according to the latest tally by Johns Hopkins University.