5 reasons experts are optimistic about Pfizer's coronavirus vaccine. 3 reasons why they're cautious.

Science and health experts are bullish on the news about the Pfizer and BioNTech coronavirus vaccine candidate.

The main reason for the excitement is the vaccine looks like it's on pace to fly right past the Food and Drug Administration's 50 percent efficacy threshold, according to its interim analysis, which pegged the rate at 90 percent so far. There's more good news between the lines, though — the companies say they haven't observed any safety concerns among trial volunteers, the immune response should last for up to a year, and it's unlikely any mutations would be able to overcome the immune response.

The analysis looks promising for Pfizer's competitors, as well, since many other candidates are targeting the same protein on the virus. Plus, if the technology behind the vaccine is successful against COVID-19, it could open a realm of possibilities for vaccine development, at large.

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That said, there are reasons to temper expectations, which, after all, are based on a press release. The 90 percent efficacy rate could drop as the trial continues (additionally, safety concerns could arise and the questions about immunity length and mutation protection remain open-ended), and there's also a lack of clarity about whether the vaccine may protect against the most severe COVID-19 cases and whether it protects elderly people. Tim O'Donnell