Dr. William Hartman has had an unusually large number of people cancel on him recently.
Hartman runs one of AstraZeneca's Phase 3 Covid-19 vaccine clinical trial sites, at UW Health in Madison, Wisconsin. But last week, a handful of trial volunteers either canceled or simply did not show up for their scheduled appointments.
"People are asking if they can withdraw from the trial," Hartman said. Although he has been able to fill empty slots so far with people on the waiting list, he said he believes the reason for the slight setback may be the apparent success of two other vaccine candidates: those made by Pfizer and Moderna.
Both companies have reported promising results from preliminary analyses of their Phase 3 clinical trials. Health and Human Services Secretary Alex Azar said during a White House task force briefing Thursday that he expects Pfizer to ask federal regulators for an emergency use authorization for its vaccine Friday. Moderna has said it intends to follow suit in the coming weeks. That would set them up to roll out the first shots in December.
Hartman said he suspects that if such authorizations are, indeed, imminent, his volunteers may choose to "wait for the vaccine that's just around the corner."
Public health officials say that is misguided.
Dr. Stephen Hahn, commissioner of the Food and Drug Administration, said he was "concerned" about any decrease in trial participation during a "Doc to Doc" interview Thursday with NBC News senior medical correspondent Dr. John Torres, which was streamed on Facebook.
Even though the data from Moderna and Pfizer are promising — both reported about 95 percent effectiveness — Hahn said the companies are far from the finish line. Regulators have not yet had a chance to review the raw data from either company.
"Until we look at the data, until we have a clear sense of efficacy and safety, these trials should continue," Hahn said. "I would encourage people to enroll."
Hartman's team at UW Health aims to enroll up to 1,000 participants. Right now, it has about 150 signed up.
It is clear that one or two vaccines ultimately will not be enough to combat the pandemic.
"If we have multiple manufacturers making multiple safe and effective vaccines, then we can vaccinate more people quickly," said Dr. Buddy Creech, an infectious disease expert and the director of the Vanderbilt Vaccine Research Program at the Vanderbilt University Medical Center in Nashville, Tennessee.
Vanderbilt has been involved in the Moderna clinical trials, and just last week it launched a separate trial for Johnson & Johnson's vaccine candidate. Creech said he has been able to avoid noticeable drop-offs of study volunteers by reminding people that it will be months before enough vaccine is available for anyone who wants it.
"The number of doses available to folks in our area will be really limited in the next four to six months," Creech said. "Go ahead and enroll in clinical trials. It's a good chance to be able to contribute further in this pandemic."
Even if the FDA greenlights a vaccine for emergency use in the coming weeks, experts reminded those eagerly awaiting the shots that such authorization is not the same as full approval.
Such an approval will require more intense scrutiny from regulators — and more data from clinical trials.
"We will not have a complete idea of safety and effectiveness" in the event of an emergency use authorization, said Norman Baylor, president and CEO of Biologics Consulting, a biotech consulting firm. Baylor is a former director of the FDA's Office of Vaccines Research and Review.
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"We have to continue and complete the studies so the FDA can make a determination on whether vaccines should be approved," he said. "We cannot rest."
Baylor said it is a "good thing" that multiple vaccine candidates are in the pipeline. Four candidates are in the middle of Phase 3 clinical trials in the U.S. Overall, more than 100 are in development or other stages of study.
Dr. Paul Kilgore, a vaccine investigator at Henry Ford Health System in Detroit, said having multiple vaccine manufacturers "provides assurance that if one manufacturer or another has to stop production even temporarily, we would have a Plan A, Plan B and Plan C."
Kilgore's team has been involved with clinical trials of both the Moderna and the Johnson & Johnson vaccine candidates. He said it is crucial to see the trials through "to the end, to make sure that we gather as much safety and immune response data and efficacy data as possible."
Realistically, for the general public, I don't think a vaccine is going to be available before April or May.
Baylor warned of other wild cards that could delay any vaccine rollout. Moderna and Pfizer have predicted that they could provide hundreds of millionsof vaccine doses over the next few months. While there is no reason not to believe such projections, it is possible for manufacturing problems to pop up. "What if you come up short?" Baylor asked.
Dr. Carlos del Rio, executive associate dean of the Emory University School of Medicine in Atlanta, urged Americans to remember that despite encouraging and welcome vaccine data, "there is no vaccine available right now."
As of Friday, spread of Covid-19 had reached unprecedented levels in the U.S., rising in every state.
"This is not a time to give up on masking" and other mitigation efforts, such as hand-washing and physical distancing. "The world is burning out there, and realistically, for the general public, I don't think a vaccine is going to be available before April or May," del Rio said.