By Mercedes Yanes Lane
No, pandemics do not last forever. In this article published in Immunity, the authors provide a description of the epidemiological and immunological measures that underlie the transition from a pandemic state to an endemic state. Measures such as immune efficacy, disease prevalence, severity of disease, and age distribution all play a role in a disease becoming endemic.
A disease is endemic when the number of cases in a population becomes stable, resulting in no quick rises or peaks of infection. This usually happens when the levels of immunity in a population are high enough that transmission declines. If immunity provides lifelong protection from infection, then widespread vaccination can generate sufficient population immunity to allow for a disease to be eliminated. However, if immunity wanes over time, then a disease can become endemic. This phase is characterized by a balance between a) people getting vaccinated, and b) newborns (not yet vaccinated and therefore susceptible), unvaccinated individuals, or by waning of immunity in previously immune individuals. It is important to note that the efficacy of the immune response to an infection is a key element of the endemic phase.
Efficacy of the immune response to SARS-CoV-2:
- Immune efficacy englobes reducing risk of infection, infectiousness (how easily an infected person can transmit the disease), and severity of the disease.
- Vaccination and previous infection confer high levels of immune efficacy.
- Immunity to SARS-CoV-2 declines over time.
- Re-infection studies suggest that people can become infected and get the disease more than once, but that subsequent infections tend to be mild. This indicates that immunity that confers protection against infection declines faster than immunity that protects against severe disease.
How long it will take for SARS-CoV-2 to reach an endemic phase depends on factors such as the virus’s effective reproductive number (i.e., how many infections are derived from one case), vaccination uptake and efficacy, and the rate at which immunity wanes. Severity of cases during the endemic phase depends on the age distribution of cases and how fast protection against infection and severity declines. If most primary infections are in children, an age group that tends to have mild forms of the disease, then primary cases are unlikely to contribute to disease severity. However, if protection against infection and severity decline at the same time, severe re-infections may occur.
For SARS-CoV-2, vaccines and infection-acquired immunity do not appear to produce life-long immunity that blocks disease transmission. Population-level immunity can only be transient, making endemicity the likely path forward for SARS-CoV-2.
Antia R, Halloran ME. Transition to endemicity: Understanding COVID-19, Immunity (2021). Immunity, 24 september 2021 doi: 10.1016/j.immuni.2021.09.019