At least four candidates are near the finish line in the U.S. coronavirus vaccine race, but how well they will work remains a question.

A key point to note, however, is that the vaccine isn’t an end-all solution to the pandemic. That’s in large part because any inoculations developed now are focused on simply preventing symptoms from arising, rather than blocking out the virus altogether.

The latter goal is a secondary endpoint, according to Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

“The primary thing you want to do is that if people get infected, prevent them from getting sick, and if you prevent them from getting sick, you will ultimately prevent them from getting seriously ill,” Fauci said at Yahoo Finance’s All Markets Summit Monday.

“What I would settle for, and all of my colleagues would settle for, is the primary endpoint to prevent clinically recognizable disease,” he said.

That level of protection would be the ultimate goal to diffusing the crisis, but is hard to do with companies facing an immediate demand for some sort of solution. While no vaccine is 100% effective, having a majority of the population inoculated and higher percentages of efficacy is the best to hope for. The most effective vaccine is the measles vaccine, which elicits an antibody response 98% of the time.

Currently AstraZeneca (AZN), developing Oxford University’s vaccine; Pfizer (PFE) with partner BioNTech (BNTX); Moderna (MRNA); and Johnson & Johnson (JNJ) are all in-late stage trials. Among them, Pfizer and BioNTech are in the lead to provide a first look at the vaccine’s effectiveness in the second half of November. At best, the duo could be the first to receive emergency use authorization and the only ones to do so by the end of the year.

But there are still other vaccine candidates in the pipeline — many in early or pre-clinical stages. If their slower trajectory puts them in a situation of testing the vaccine when the outbreak lessens, it could make it harder to test the potential effectiveness of a candidate.

For this reason, the U.K. is looking at challenge trials, which intentionally infect a smaller group of participants with the virus in an effort to test a vaccine’s or treatment’s efficacy.

Fauci said the U.S. is not anticipating such a move because the rate of spread is so high in the country that it’s sufficient enough of an environment to test the vaccine. The daily case count for the U.S. in the past few days has reached a new record of more than 80,000 cases.

“So although you can get some good information from a challenge trial, the real-world information that you want is out in the field when someone is actually being exposed to natural infection, and to determine if the vaccine prevents against that,” Fauci said.

“So right now we're not planning any challenge studies because we have so much infection going on.”

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