Public health experts, even optimistic ones, say it may take months after a COVID-19 vaccine is developed before public and economic activity begins to resemble pre-pandemic days.

“I think people have construed the development of a vaccine as almost like a reset button that will take us back to life prior to the pandemic,” said Yonatan Grad, a professor of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health. “I am worried we’re not going to be able to just press a button and go from pandemic to no pandemic. It will take a lot of time, and it may not happen to quite the extent that we might imagine.” 

Enormous hopes are hinging on the vaccine race that could blunt the coronavirus’ impact. At least 142,401 U.S. residents have died of COVID-19, according to the Johns Hopkins Coronavirus Resource Center. Millions of people have lost their jobs.

But stopping transmission is more complicated than approving a vaccine.

“The media will describe this as a magic bullet, but there’s a difference between vaccine and vaccination,” said Paul Offit, chief of infectious diseases at the Children’s Hospital of Philadelphia.

Epidemiologists think about achieving herd immunity as a formula with three variables: how infectious the disease is, the effectiveness of the vaccine and how many people receive the vaccine. At least 70 percent to 85 percent of the population must have immunity either through prior infection or through a vaccine to beat this novel coronavirus, the Trump administration’s top scientists estimated at a congressional hearing in late June.

“It’s really got to be over 70 percent of the population that’s got to be immune before we even begin to see any impact on herd immunity,” said Centers for Disease Control and Prevention Director Robert Redfield.

“I would say 70 at the lowest,” said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

That’s going to take some time, according to virologists, vaccinologists and regulatory experts. Success is not guaranteed.

Headlines heralding incremental progress toward a vaccine paper over unanswered questions, such as how effective a vaccine will be, how long it will take to manufacture sufficient doses and how many shots will be needed. It’s unclear whether the vaccine will be as effective in vulnerable populations such as older Americans or how long protections from a vaccine will last.

Then there are questions about uptake. How many people will refuse to get one? How much will it cost?

“Getting to high rates of COVID vaccination won’t be like flipping a switch,” said Jason L. Schwartz, assistant professor at the Yale School of Public Health’s Department of Health Policy and Management. “With any new vaccine, it can take years to get anywhere close to the 70 percent to 80 percent targets we’re hoping for with these vaccines.”

The Trump administration has promised a vaccine will be ready before January.

“We’re going to make sure we have a vaccine by the end of the year for emergency use,” Treasury Secretary Steven Mnuchin said.

Other vaccination efforts

But vaccinating enough people to stop transmission will take many months more.

In 2009, during the H1N1 flu epidemic, a vaccine was distributed for free, with the government buying and distributing the supply. The World Health Organization declared H1N1 a public health emergency of international concern in April 2009. 

Although the process was slowed by an early manufacturing error, 124 million doses were distributed from October 2009 to January 2010. By the end of May 2010, about 90 million doses had actually been administered.

Jesse Goodman, who formerly led the Food and Drug Administration’s Center for Biologics Evaluation and Research, estimates the distribution of a COVID-19 vaccine will be similar, taking about four to six months.

Getting people vaccinated is another matter. And it’s unclear how effective a COVID-19 vaccine will be.

It took more than 30 years to eradicate measles, a disease with about 3 million to 4 million cases each year, with a vaccine that has about 97 percent efficacy, Offit said. Measles is more infectious but less deadly than COVID-19.

A vaccine for haemophilus influenzae, a disease with about 20,000 cases each year, was developed in 1987 with 95 to 100 percent efficacy, and cases had nearly disappeared by 1995.

Polio vaccines were introduced in the U.S. in 1955 and 1963, and the number of polio cases declined to dozens in the 1960s and single digits in the 1970s, according to the CDC.

The FDA has said in guidance for the pharmaceutical industry that clinical trials should show a vaccine is at least 50 percent effective, but it will face enormous pressure to make a vaccine available ahead of its full approval process through an emergency use authorization.

“A lot of that will depend on how bad the flu season is this winter and the political environment. This is all happening in the middle of a highly contentious presidential election,” said Gregory Poland, an internist at the Mayo Clinic and a spokesman for the Infectious Diseases Society of America.

There is still also a lot to be learned about whether a COVID-19 infection protects you from illness and, if so, for how long.

Observational studies suggest protective antibody responses began to deplete in patients within weeks of COVID-19 symptoms beginning to show, depending on the severity of the disease. That has implications for the durability of any vaccine.

Natural infection also produces memory B-cells and memory T-cells, which could also confer some protection against COVID-19. And it is possible to have a vaccine that confers longer protection than natural infection, Offit said.

But a lot about SARS-CoV-2, the coronavirus that causes COVID-19, is unknown, and some of the technologies the United States is investing in are largely unproven.

“We began making influenza vaccines in the 1940s. Eighty years later, clinical trials are still happening on how to make a better flu vaccine. We’ve known about the flu for more than 80 years. We’ve known about SARS-CoV-2 for only a few months,” said Poland.

The human papillomavirus vaccine, which has 90 percent efficacy and was approved in 2006, still does not have anything close to universal uptake, and it hasn’t stamped out HPV, Schwartz said.

Schwartz described the typical process of vaccinating a population as a “slow climb.”

“Sometimes we don’t get there,” he said.