What will it take to make COVID-19 vaccine effective?

Using a computer simulation model, scientists in the United States have assessed the effectiveness and coverage levels a potential COVID-19 vaccine needs to have in order to completely extinguish the disease spread, an advance that may help shape expectations for policy makers.

The study, published in the American Journal of Preventive Medicine, evaluated the impact of introducing a vaccine in the US with varying abilities to protect against infection without other measures, such as social distancing, in place.

According to the scientists, including those from the CUNY Graduate School of Public Health & Health Policy in the US, the overall goal of the research was to identify the vaccine efficacy thresholds above which vaccination could prevent a wave of the epidemic, or extinguish an ongoing epidemic.

They assumed a range of possible scenarios to carry out the simulation rather than predict exactly what will happen with the current pandemic.

"The aim was to represent the spectrum of possibilities if social distancing measures were relaxed completely," the researchers wrote in the study.

If only 60 per cent of the population gets vaccinated, the study noted that the efficacy of the vaccine should be around 80 per cent to prevent an epidemic, and 100 per cent to extinguish an ongoing epidemic.

The simulation experiments carried out by the scientists revealed that to prevent an epidemic, the vaccine efficacy has to be at least 60 per cent when the immunisation coverage is 100 per cent.

In the research, the scientists assumed the reproduction number, which is the number of people that a person carrying the virus goes onto infect, as between 2.5 and 3.5.

They said the vaccine efficacy threshold rises to 70 per cent when coverage drops to 75 per cent, and up to 80 per cent when coverage drops to 60 per cent for an assumed reproduction number of 2.5.

According to the study, when coverage drops to 75 per cent for a reproduction number of 3.5, they said the efficacy rises to 80 per cent.

In order to extinguish an ongoing epidemic, the scientists said the vaccine efficacy has to be at least 60 per cent when the coverage is 100 per cent, and at least 80 per cent when coverage drops to 75 per cent to reduce the peak.

They said the reduction under this scenario happens by 85–86 per cent, 61–62 per cent, and 32 per cent when vaccination occurs after 5, 15, and 30 per cent of the population, respectively, have already been exposed to the deadly virus.

The study noted that a vaccine with an efficacy between 60 and 80 per cent could still obviate the need for other measures under certain circumstances such as much higher, and in some cases, potentially unachievable, vaccination coverages.

"Some are pushing for a vaccine to come out as quickly as possible so that life can 'return to normal.' However, we have to set appropriate expectations," said study co-author Bruce Y. Lee from the CUNY Graduate School of Public Health and Health Policy in the US.

"Just because a vaccine comes out doesn't mean you can go back to life as it was before the pandemic," Lee said.

He cautioned that for vaccines, like many other products, "what matters is not just that a product is available, but also how effective it is."

The scientists believe the study's findings can provide targets for vaccine developers as well as shape expectations for policy makers, business leaders, and the general public.