Attitudes Toward a Potential SARS-CoV-2 Vaccine

A Rush to Vaccine Hesitancy

There is a complex calculus in building the immunity across a population known as the herd effect. In simple terms, I consider the interplay of naturally acquired immunity—gained from infection and recovery—and vaccine-induced immunity. The amount of vaccine-induced immunity is further dependent upon the effectiveness of a vaccine and the percent of the population (herd) receiving the vaccine. Complicating the estimate is the durability of the immune response.

In the diagrammatic example presented here, a hypothetical 65% threshold for the herd effect could be attained with 15% naturally acquired and 50% vaccine-induced immunity. And herein lies the challenge. To reach that 50% level, a vaccine would need to be—on average—about 70% effective, and about 70% of the population would need to be willing to roll up their sleeves and be vaccinated.

For those following the advent of COVID-19 vaccines, the results of a recent Kaiser Family Foundation tracking poll should give pause:

“… if a coronavirus vaccine was indeed approved by the FDA before the election and was made available and free to everyone who wanted it, just four in ten adults (42%) say they would want to get vaccinated while a slight majority (54%) say they would not want to get vaccinated under those circumstances.”

An uptake of 42% will not get us where we need to be… even with an unprecedented 100% vaccine effectiveness. Accordingly, there is an unparalleled need for the complete assessment, through well-designed clinical trials, of safety and efficacy of candidate vaccines. Following such assessment, there is an equally significant need to assure the public that any COVID-19 vaccine is safe and has undergone the highest levels of scrutiny.

The thing to keep in mind with the clinical trials used for vaccines is that there are rolling enrollments of participants. As time goes on, more and more people are recruited and randomized into receiving vaccine or placebo.

Following this, we wait. We wait for people to become exposed to infection and become ill. We wait for adverse events to occur. The researchers only control the exposure (when people receive the vaccine or placebo). They do not control the outcome (infection/illness/adverse event).

The longer we wait, the more we are able to accumulate the outcome events of interest and the more precise our estimates become of the true effect of a vaccine. Early evaluations of most vaccines provide very imprecise estimates of their effects. Accordingly, there are very good reasons to wait for sufficient data to accrue so as to improve our estimates.

For example, an early assessment of a vaccine's efficacy at preventing a disease might be 50%, but with a confidence interval that goes from 30% to 70%. That means we could jump in to a program with an intervention that provides less benefit than desired. Likewise, for less common side effects, we could convince ourselves that a vaccine is quite safe when it may not be so.

Time for ascertainment of effect is a very important concept. Like a good wine, time has a way of improving the products of well-designed clinical trials.

Reference

1. Hamel L, Kearney A, Kirzinger A, et al. KFF health tracking poll - September 2020: top issues in 2020 election, the role of misinformation, and views on a potential Coronavirus vaccine. Published: Sep 10, 2020. https://www.kff.org/coronavirus-covid-19/report/kff-health-tracking-poll-september-2020/