Dr. Mark Kortepeter, a physician and biodefense expert who formerly worked at the U.S. Army “hot zone” research lab, explains the math behind how Covid-19 death tolls are overtaking deadlier viruses. In the pantheon of infectious disease killers, Ebola virus stands out as one of the deadliest. The Zaire species of Ebola kills somewhere between 40% to 90% of its victims, and usually upwards of 60% of infected people die. Only a handful of infectious diseases can claim such high death rates, including rabies, pneumonic plague, and inhalational anthrax. The SARS-CoV-2 virus that causes Covid-19 illness surpassed a grim milestone in early July. The number of deaths from Covid-19 in Africa—more than 11,950—exceeded the total number of people who died during the largest-ever Ebola outbreak in West Africa, according to the World Health Organization.
How could this be? How could a disease that usually kills greater than 60% of its victims be outgunned by Covid-19, which “only” kills about approximately 4% of its victims. The answer relates to one fundamental aspect of most viruses. They don’t really like to kill their hosts. They can have a much wider impact if their hosts don’t die. This allows them to circulate in the community much longer and spread far and wide across the world for far greater impact. When someone becomes ill with Ebola virus, they become bedridden very quickly. It’s really hard to be out in the community spreading disease if you are vomiting or having massive diarrhea. The people who are at greatest risk for Ebola infection are those who have very close contact taking care of the sick, bedridden victims—whether they are in the home or the hospital. Add to that, Ebola virus doesn’t spread until the victim has symptoms. This makes determining who is infected with Ebola and deciding who to isolate and quarantine much simpler than with Covid-19. Some infected people can spread Covid-19 without symptoms or before they have symptoms. Such individuals won’t be bedridden at the time they are contagious. Instead, they can hang out at a beach party or in a bar and be spreading the virus without even knowing they have been infected. The more contagious a human host is, and the more social interactions they have in the community, the more opportunities there are for a virus to spread during those interactions, and the more it will spread. This is the bedrock behind the measures that public health authorities have been championing since the beginning of the Covid-19 pandemic: quarantine (restricted movement of those exposed, but not yet ill), isolation (restricted movement of those who are ill), masks (to reduce chance for spread, since we don’t know by looking at someone whether they are infected), and social distancing (to minimize close interactions between people). Ultimately, though, a disease like Covid-19 that has a lower fatality rate, on average, in a single patient, can actually kill more people based on simple math. More people infected translates to more deaths. This is where Covid-19 wins hands down, and the reason that the recent surge in cases across parts of the United States is so dangerous. The more people are infected and contagious, the more people will die.
Here’s a simple calculation that explains this:
In the largest Ebola outbreak in West Africa, there were 28,616 cases of Ebola virus disease and 11,310 deaths, for a death rate of 39.5% (low compared to historic death rates for Ebola Zaire).
If we only had 28,616 cases of COVID-19, at the current death rate of 4.1%, that would translate to 1,173 deaths. But, when you have nearly 600 times as many cases across the world (more than 17.1 million at the time of this writing), because SARS-CoV-2 virus spreads efficiently and we have failed to contain it, even though the death rate is “only” around 4%, that translates to the current number of 669,000 deaths worldwide. As I have said before, infectious disease epidemiology is not rocket science. A ninth grader doing basic algebra could perform that same calculation. Other countries have figured this out.