COVID-19 Vaccine Success Will Depend On Factors Beyond Efficacy Rate, Harvard Medical School Study Finds


A team of researchers at Harvard Medical School has found that the success of a COVID-19 vaccine will depend on not only its efficacy, but also the pace at which it is delivered, the severity of the pandemic, and the public’s acceptance of immunization.

Three major pharmaceutical companies released phase three clinical trial results for their COVID-19 vaccine candidates in November, reporting vaccine efficacy rates — the percentage reduction of disease in the vaccinated group compared to the unvaccinated group — well above 50 percent, the threshold for approval by the Food and Drug Administration.

The study, published in Health Affairs on Nov. 19, was a collaboration with researchers at Yale School of Public Health led by Rochelle P. Walensky, a Harvard Medical School professor and the Chief of Infectious Diseases at Massachusetts General Hospital — a Medical School affiliate.

Walensky said the group was motivated by a desire to understand how these vaccine efficacy rates translate and contribute to their true effectiveness in the general population.


“There's a lot of different definitions to vaccine efficacy,” Walensky said. “What does that mean? Does that mean that the person who receives the vaccine doesn't get the disease? Does that mean they can't transmit disease? Does that mean they don't get severe disease?”

The team sought to understand true vaccine effectiveness — how well a vaccine works in the population — by accounting for factors in the “current backdrop” of the pandemic which affect vaccine implementation, according to Walensky.

“We combined all those parameters in this model that we were building to look at not only efficacy, but also parameters of effectiveness like coverage, pace, and how bad the pandemic is at the time you're trying to do a vaccination strategy,” Walensky said.

Walensky said the researchers also considered the Americans’ willingness to get vaccinated — a key factor in planning, given that recent surveys have found many would not seek vaccination if a COVID-19 shot were to become available.

The proportion of the population that receives a vaccination is just as important as the vaccine’s efficacy, Walensky said.

“Say you have for all intents and purposes a 100 percent efficacious vaccine but it takes six months to roll out to 50 percent of the population,” she said. “If you only get it to 50 percent of the population, your coverage drops to 50 percent immediately.”

Though the federal government has invested billions in vaccine development through its Operation Warp Speed program, only millions have been invested in implementation and scale-up, according to Walensky.

“The investments in development have not nearly been matched by orders of magnitude in the investment in deployment and communication,” Walensky said.”While there have been a lot of plans that have been drafted, they're not necessarily funded plans, and those plans really need to have money behind them.”

Walensky also said that even a highly effective vaccine will struggle to control the pandemic if infection rates continue to rise.

“If you have a cup of water and it can put out a fire on your stove, that same cup of water can't put out a forest fire,” Walensky said. “It really may work very well, but it's only as good as the backdrop in which it is trying to work.”

Practicing social distancing and mask wearing is key to lowering infection rates and creating the best environment for maximizing vaccine effectiveness, according to Walensky.

“We should not be giving this vaccine more work than it already has to do, and by putting it in the backdrop of a lot of cases, we are doing that,” she said.

—Staff writer Virginia L. Ma can be reached at