Pandemic numbers misleading without context
Since the start of the pandemic, public health authorities have been fastidiously counting the number of people infected with the coronavirus. For both the medical profession and the media, these rising figures have been the principal way of framing the pandemic in the U.S.: “124,000 new cases a day,” “802,000 COVID deaths since February 2020.” But this information offers an incomplete picture of the crisis, potentially warping the public’s understanding in ways that could prolong the pandemic and even add to its toll.
What’s missing is the number of uninfected people and the number of infected people who survive COVID-19. That provides a denominator. If there were 124,000 new infections per day, how many people were exposed? If 802,000 people died from COVID, how many were infected but didn’t die?
Such information is the most underreported story of the pandemic. But it has long been an important piece of public health information. It advances our understanding of the nature of the disease; it hints at the power of precautions; and it can allay fears and trauma that people are experiencing about the seemingly never-ending nature of the pandemic.
Our reliance on numbers to understand epidemics can be traced to the development of epidemiology — when medical and scientific authorities had not yet uncovered how microbes caused the spread of infectious disease. Between 1755 and 1866, when epidemiology emerged, medical practitioners believed that environmental factors caused disease. Based on this inaccurate view, they had few effective metrics to understand the origins of epidemics. As such, they counted the number of uninfected and infected patients; the number who contracted a disease and the number who died; they examined those who were hospitalized and those released.
Statistics, and exploring the behaviors behind them, became a key component in epidemiological analysis because that’s all that health experts had — and it helped them craft treatment strategies.
In response to a cholera outbreak in India, William Twining, a British military doctor there, published an influential comprehensive volume on diseases in 1832. The treatise provided copious detail of hospital attendants who came into close contact with cholera patients and soiled linens but did not become ill. Had the text focused on people who became sick, a reader might have been misled about the risk of the disease, or led to look for its causes in the wrong place. With context about the unafflicted, the study offered key evidence that cholera was not transmitted through direct contact.
It was another set of counterexamples two decades later that helped the young science of epidemiology to zero in on the culprit. John Snow, a physician in London, famously found the common denominator among cholera cases in an 1854 outbreak: Those who became sick seemed to all have drunk water from a pump in the center of a poor neighborhood. Cementing his conclusion was the fact that employees at a nearby brewery, which had its own pump, did not contract cholera.
Learning about the daily lives of these brewery workers led Snow to theorize that cholera was transmitted through contaminated drinking water. To understand how a disease spread, he was equally invested in the infected and the uninfected.
As epidemiology evolved, medical authorities continued to consider the uninfected by developing a new statistic: incidence rate or attack rate, which is still used today. This refers to the number of new infected cases within a specific period measured against the population. While epidemiologists tabulate this rate, the media does not typically broadcast it. Instead, we are inundated with infection and death rates. Reporting the number of infected is a numerator. We are missing the denominator.
A recent example shows why the missing denominator is important: This past summer, the media jumped on one of the first major outbreaks of breakthrough cases in Massachusetts. This provided epidemiologists with valuable evidence of how the delta variant infected many vaccinated people — but no one actually counted the number of people who were exposed but not infected. (To be fair, documenting exposure among uninfected people is more challenging.)
By focusing on the vaccinated who became infected, the media inadvertently gave the impression that the delta variant had superpowers. If it is super, it also has a weakness: the vaccines. That emerges if one counts the uninfected and looks at vaccination rates. A narrower focus risks overplaying the danger of the variant and underplaying the value of the vaccines.
Epidemiology needs to remember its roots and school the public.