CNN  — 

Baseball great Yogi Berra once said, “It’s tough to make predictions, especially about the future.”

Yogi Berra didn’t know anything about Covid-19 – he passed away five years ago – but his quote applies to the development of a Covid-19 vaccine.

Over the past six months, pharmaceutical companies have made various predictions about their Covid-19 vaccine timelines that have turned out not to be true.

In one recent example, Pfizer has said for weeks it would know by the end of October if its vaccine works or not – but Tuesday on an investor call, the company’s CEO essentially ruled that out.

While at times Pfizer and other companies have couched their statements, other times they have been more definitive about their projections.

Scientists say that should guide us as we move closer to having a vaccine: Don’t believe everything you hear, because testing and manufacturing vaccines is notoriously unpredictable.

“Unexpected things happen all the time in vaccine development,” said Dr. Nelson Michael, an Army vaccine specialist who has worked on more than 20 vaccine clinical trials. “There are tons of twists and turns, and it’s important to understand that.” 

While health officials have also made forward-looking statements, they’ve typically been more vague than pharmaceutical companies.       

Last week, National Institutes of Health Director Dr. Francis Collins told the National Press Club he remains “cautiously optimistic” that the United States could have a Covid-19 vaccine authorized by the end of the year, but warned “it might take longer.” 

Dr. Paul Offit, a member of the US Food and Drug Administration vaccine advisory committee, said pharmaceutical companies would be wise to stop making forecasts about their timelines.

“Companies should stop making predictions, because nature is very humbling,” Offit said.

Claim of a ‘near perfect’ vaccine   

In September, Ugur Sahin, CEO of BioNTech, which is working with Pfizer on its coronavirus vaccine, told CNN that his company’s vaccine is “near perfect.”

Scientists interviewed for this story shuddered at the thought of describing a vaccine as “near perfect” when it hasn’t yet been fully studied in large-scale trials. The Pfizer vaccine, as well as three others, are still in Phase 3 clinical trials in the United States, and no one knows if they work at all – let alone to near perfection.

Vaccines by Pfizer and another pharmaceutical company, Moderna, utilize a new kind of technology in their Covid-19 vaccines – no vaccine on the market has ever used it.

Offit said that alone is reason for caution. 

“This virus has been around for less than a year, and it causes a variety of clinical findings that we never would have predicted, and now we’re going to counter it with a vaccine that has no commercial experience? How about a little humility here?” said Offit, a member of the FDA’s Vaccines and Related Biological Products Advisory Committee.

According to a BioNTech statement sent to CNN, Sahin’s comment was “based on preliminary antibody and T cell responses and a favorable safety profile observed in the study so far. He also noted that there is a need to wait for the efficacy data which is not available yet.”

Pfizer’s CEO has made predictions about when it will be clear if a vaccine works or not.

On September 8, Albert Bourla told the Today show that “we will have an answer by the end of October” whether the vaccine works, adding that “our model, our base case, predicts that we