Between January 1918 and December 1920, an influenza pandemic circled the world and killed up to a quarter of the world’s population. It only ended when the virus ran out of susceptible people to infect. At the time, there were no vaccines against influenza, no antiviral medications and no antibiotics for those who developed secondary bacterial pneumonia. In epidemiology, we look at these epidemics and explore the different possibilities under which they will end. Coronavirus is not different from what happened in 1918 in terms of what we are dealing with: a novel respiratory virus that is transmissible from person to person.
The first scenario under which the pandemic ends is the grimmest scenario because everyone dies. If everyone is killed by the virus, then there are no more susceptible people. Quite literally, it is the end of the story. Not many viruses or bacteria would do this because high lethality means less of a chance of the pathogen (the germ) to continue infecting. It would also trigger nearly immediate action to contain it. Because the bubonic plague (Yersinia pestis) was highly lethal, very severe control measures were put into place. It’s how we got the term quarantine, from the 40 (cuarenta) days sailors had to stay on their vessels off the Italian ports before they could disembark. If they didn’t have the plague by the end of the 40 days, the vessels was considered to not be carrying it.
The second scenario is less grim, but still troublesome. In this scenario the virus either kills its victims or the survivors are rendered immune. This eliminates the susceptible people while leaving plenty of people alive. Humanity survives and continues to evolve. This may have happened in the 1918 pandemic in most places. But it is more likely that the third scenario ocurred.
In the third scenario, the virus either kills its victims or the survivors are rendered immune, like in the second scenario. However, in this scenario not everyone gets infected. Those who are not infected and not immune — but still susceptible — are protected by community immunity (aka “herd immunity”). That is, those who were sick but recovered and became immune surround the susceptible people and protect them from the virus by blocking the chain of transmission. This, however, is not the best scenario.
The fourth scenario is like the third one, but immunity is gained by almost everyone not through disease alone but mostly through immunization. This is better than the third scenario because people don’t have to go through the disease to become immune. For this, the vaccine has to be highly efficacious so that the most number of people are no longer susceptible. Also, the infection can’t be so fast that there is no time to create the vaccine, test it and deploy it.
The fifth and final scenario is that we isolate people as quickly as possible and cut the chain of infection through social distancing, isolation and quarantine. For this to work best, we need a fast, affordable, and reliable laboratory test. And we would need everyone to be tested. In this context, social distancing involves people staying away from each other, not frequenting public spaces and reducing their exposure to others. Isolation means that people who are infected are isolated from others for the duration of their symptoms plus whatever time the evidence shows that they are capable of transmitting the disease. Quarantine, as mentioned above, is keeping people who are not ill but may have been exposed, away from others until enough time has passed and no symptoms appear.
Of course, there are other scenarios out there that are different from these because they involve diseases with different modes of transmission or different outcomes. The HIV pandemic, for example, is now into its fourth decade. HIV was slow enough to spread because of a very long period between infection and symptoms that allowed those infected to not know they were infected. Cholera, of which there have been several pandemics, is spread mainly through contaminated water, so places with environmental controls can avoid it without the need for immunity to necessarily come into place.
As the coronavirus pandemic continues, public health workers around the world along with scientists and medical personnel are working around the clock to prevent the first scenario… What scenario ends up being the one that brings the coronavirus pandemic to its end is yet to be known.