A vaccine could mean a turning point in the fight against a novel coronavirus that has infected at least 7.8 million people in the United States, killed more than 214,000 nationwide and unraveled aspects of most everyone's lives.
But approving a vaccine quickly is complex. And distributing it to some 330 million Americans, and to other countries, will require a feat of logistics.
Here's what you need to know about the COVID-19 vaccine effort.
What's the timeline?
It depends on two main factors: how long the vaccines take to develop and get approved, and how quickly they can be manufactured and distributed.
Experts say multiple vaccines are needed to immunize the U.S. population. Clinical trials on several vaccine candidates could soon have produced enough data to assess whether they're effective.
Moderna and Pfizer are farthest along, with vaccines deep into phase 3 clinical trials involving tens of thousands of people, according to Moncef Slaoui, the scientific head of Operation Warp Speed, a federal program working toward immunizing Americans as quickly as possible.
Both are mRNA vaccines, which introduce genetic code for viral antigens that could teach the body's immune system to respond to the virus. This new platform has yet to produce an approved vaccine of any kind.
Slaoui, who spoke Oct. 6 at a virtual event co-hosted by the University of Washington and Johns Hopkins University, said he expected "efficacy outcomes" -- data on whether the vaccines work -- within the next seven weeks.
The federal government also is investing in three other platforms besides mRNA, including adenovirus-based vaccines and adjuvanted protein vaccines, which could reach efficacy sometime next year.
Scaling up the manufacture of vaccines is "equally important and, frankly, even more complicated" than clinical development of them, Slaoui said. He discouraged vaccine makers from filing for emergency approval until their vaccines are available to distribute.
The mRNA vaccines already are being stockpiled, pending trial results and regulators' approval.
Slaoui said he felt comfortable that within two months, one or both mRNA vaccines would reach efficacy and would have enough already stored to vaccinate 30 million Americans by year's end and 50 million more in January.
That's an optimistic timeline.
Dr. Robert Kadlec, Health and Human Services (HHS) Assistant Secretary for Preparedness and Response, said the U.S. can expect vaccine delivery starting in January.
And Dr. Anna Wald, a UW professor of medicine, epidemiology and laboratory medicine, said that even if limited quantities are shipped later this year, she didn't expect vaccines to impact the pandemic's course until spring 2021.
Because the trials are so large, data accumulates quickly, and scientists stand to learn more about how effective the vaccines are the longer they wait to review results, said Dr. Larry Corey, former director of the Fred Hutchinson Cancer Research Center and co-leader of a national network coordinating clinical trials on vaccines.
"We will learn a hell of a lot more" by waiting until December, Corey said. "We'll have a lot of doses in January and February, so why don't we wait and do it right?"
Dr. Keith Jerome, head of the virology division in UW Medicine's Department of Laboratory Medicine, said researchers believe 60-80% of the population will need to be protected from infection to reach herd immunity, when the virus no longer readily transmits through communities.
"It will probably be the middle of next year before we really get to the situation where we have vaccine-induced herd immunity," Jerome said.
How will the FDA approval process work?
The FDA plans to authorize these vaccines on an emergency basis, a process less stringent than full licensure.
As hundreds of people die each day of COVID-19, there is tension between getting vaccines out quickly but also ensuring vaccines are effective and safe.
"There is a balance here," Dr. Peter Marks, who directs the federal Food and Drug Administration's Center for Biologics Evaluation and Research, said during the Oct. 6 UW event. "We'll have a narrower amount of safety data than we would have for a normal approval."
The FDA on Tuesday published criteria vaccine makers should meet before filing for an emergency-use authorization.
The agency wants to see data from randomized, double-blind studies showing the vaccine is at least 50% effective.
"We'll need to see data from a large, well-designed Phase 3 clinical trial that shows clear and compelling evidence of effectiveness," Marks said, adding that data will be reviewed by the FDA and vetted publicly.
Jerome said reviewing data will be crucial: "We need to understand what the vaccine does, in terms of preventing you from getting the disease, or if you do get the virus, how much are you shedding?"
The FDA guidance says vaccine makers should have followed people participating in clinical trials for a median of at least two months to rule out major side effects.
The White House objected to that guideline, wanting a faster pace.
Still, two months is a compressed timeline.
For licensure, "by the time the application goes in, we have more than two years or three years of data," Wald said.
Wald said the FDA's emergency guidelines were "reasonable" and should ensure the public has enough data on safety and efficacy.
The FDA also will require vaccine makers to monitor for safety and effectiveness after getting an emergency-use authorization.
Corey said scientists working on the vaccines remained independent and committed to ensuring vaccine development and the approval process could be trusted.
"We're not going to listen to the administration. We're going to listen to the data."
Why more than one vaccine?
Across the world, some 190 vaccines are in development, according to the National Academies of Sciences, Engineering, and Medicine.
The federal government already owns millions of doses of six potential vaccines, according to HHS. That approach offers more chances to find a vaccine that works.
Also, "we can't rely just on one vaccine to manufacture enough doses," Wald said.
Some vaccines could work better for particular groups of people, such as older adults with weaker immune systems. Some vaccines will be easier to store or transport.
The mRNA vaccines require two doses and cold storage, which complicates distribution logistics.
Multiple vaccines also will help ensure a supply outside the U.S.
Who will get the first vaccines?
Vaccines could take months to distribute, leaving some people waiting.
At the federal government's request, the National Academies built a framework that anticipated scarcity, aimed for equality and maximum impact, and considered the disproportionate impacts of the virus on certain populations, Dr. Helene Gayle, president and CEO of The Chicago Community Trust, said during the UW event last week.
According to the plan: Vaccine doses should initially go to health care workers and first responders who are at high risk. Then, distribution should focus on people with underlying conditions that put them at significant risk and on older adults living in congregate settings such as nursing homes.
The next priority group includes K-12 teachers and school workers, workers in critical industries, people in homeless shelters, incarcerated people, older adults, and those who have underlying conditions and are at moderate risk.
Next, young adults, children and some workers in key industries could receive the vaccine. Then, everyone else.
The committee said the vaccine should be free and distributed in a manner that acknowledges systemic racism and the socioeconomic factors that have allowed COVID-19, the illness caused by the coronavirus, to disproportionately affect some communities.
"We need to enroll people of those communities into clinical trials and we need to work to make sure the vaccine is available in those communities and people are willing to take it," Wald said.
Convincing enough people of a vaccine's safety and value could be a stiff challenge for health officials.
More than one-third of Americans say they wouldn't take a no-cost, FDA-approved vaccine, according to an August Gallup poll. A larger proportion of nonwhite Americans said they would decline.
"Histories of medical research exploitation, such as during the Tuskegee syphilis study, fuel skepticism in minority communities," the National Academies report says, referring to unethical and harmful research conducted on Black men in Alabama between 1932 and 1972.
The report added that anti-vaccine groups have been effective spreading their views online.
What's Washington state's role in distributing a vaccine?
The Trump administration's initial blockade of the FDA vaccine guidelines concerned some health leaders, who feared vaccine credibility could take a backseat to speed.
Washington state health officials say they want to see adequate clinical data and independent review of vaccine candidates.
"We'll be watching the FDA approval process closely to make sure it is thorough and transparent," said Michele Roberts, an acting assistant secretary for the state health department.
Roberts said state leaders are planning vaccine distribution, working to enroll health providers to administer vaccines and also working on logistics like determining how much ultracold storage is available in Washington.
The state owes a detailed distribution plan to the federal Centers for Disease Control and Prevention by Friday, Oct. 16.
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