COVID-19 Will Become Endemic in the Global Population – Mostly Childhood Disease Like Common-Cold

COVID-19 risks may shift from older adults to younger children as the SARS-CoV-2 virus becomes endemic, according to new modeling results.

In the coming years, if the SARS-CoV-2 virus becomes endemic in the world’s population, COVID-19 could begin to behave like other common cold coronaviruses, especially affecting young children who have not yet been vaccinated or exposed to the virus, according to to new model results. Because the severity of COVID-19 in children is generally lower, the overall burden of this disease is expected to decrease.

“After infection by SARS-CoV-2, there is a clear indication of progressively more serious consequences and fatalities with age,” said Ottar Bjornstad. “Yet our model results suggest that the risk of infection is likely to shift to younger children as the adult community becomes immune, either through vaccination or exposure to the virus.”

Bjornstad explained that such shifts have been observed in other coronaviruses and influenza viruses as they emerged and then became endemic.

“Historical data on respiratory disease indicate that patterns of age incidents during virgin epidemics may be very different from endemic circulation,” he said. “For example, ongoing genomic research suggests that the 1889-1890 pandemic, also known as the Asian or Russian flu — which killed a million people, mostly adults over the age of 70 — may have been caused by the emergence of the HCoV-OC43 virus, which is now an endemic, mild, repetitively infecting cold virus that mainly affects children aged 7-12 months.”

However, Bjornstad cautioned that if immunity to SARS-CoV-2 reinfection in adults declines, the disease burden in that group could remain high, although previous exposure to the virus would reduce disease severity.

“Empirical evidence from seasonal coronaviruses indicates that prior exposure only confers short-term immunity to reinfection, allowing for recurrent outbreaks. This prior exposure may prime the immune system to provide some protection against serious illness,” said Bjornstad. “However, research on COVID-19 shows that vaccination offers stronger protection than exposure to the SARS-CoV-2 virus, so we encourage everyone to get vaccinated as soon as possible.”

The US-Norwegian team developed what is known as a “realistic age-structured (RAS) mathematical model” that integrates demographics, the degree of social mixing, and the duration of infection-blocking and disease-lowering immunity to explore possible future scenarios for age. incidence and mortality burden for COVID-19.

Specifically, the researchers examined the disease burden in the short, medium and long term – 1, 10 and 20 years, respectively. They also examined the burden of disease for 11 different countries — including China, Japan, South Korea, Europe, Spain, the United Kingdom, France, Germany, Italy, the United States, Brazil and South Africa — which differed widely in their demographics. For each of these countries, they used data from the United Nations to parameterize the model.

“Regardless of immunity and admixture, population-level mortality may differ from country to country due to different demographics,” said Ruiyun Li, a postdoctoral researcher at the University of Oslo. “Our overall modeling framework allows robust predictions of age-related risk in light of short- or long-term protective immunity, reduction of disease severity given previous exposure, and taking into account the range of countries with their different demographics and social mixing patterns. .”

According to Li, it is well documented that social distancing affects transmissibility, and many countries have implemented interventions such as ‘shelter in place’ during the build-up to the virginal COVID-19 epidemic. Therefore, the team model assumes that the reproduction number (R0) — or the level of portability — on a given day is linked to the amount of mobility on that day. The model also includes a variety of immunity scenarios, including both independence and dependence on disease severity from previous exposure, as well as short-term (either three months or one year) and long-term (either 10 years or permanent) immunity.

The team’s results were published August 11, 2021 in the journal Science Advances.

“For many infectious respiratory diseases, the prevalence increases in the population during a virgin epidemic, but then decreases in a decreasing wave pattern as the spread of infection unfolds over time toward an endemic equilibrium,” Li said. “Depending on immunity and demographics, our RAS model supports this observed trajectory; it predicts a strikingly different age structure at the start of the COVID-19 epidemic compared to the eventual endemic situation. In a scenario of long-term immunity, permanent or at least 10 years, the young are predicted to have the highest infection rates as older individuals are protected from new infections from previous infection.

Jessica Metcalf, an associate professor of ecology, evolutionary biology and public affairs, Princeton University, noted that this prediction is likely to hold true only if reinfections cause only mild illness. However, she said mortality may remain unchanged over time if primary infections do not prevent reinfections or reduce serious illness in the elderly.

“In this bleak scenario, the excess deaths from persistent severe reinfections resulting from declining immunity will continue until more effective pharmaceutical tools are available,” she said.

Interestingly, due to variations in demographics, the model predicts different outcomes for different countries.

“Given the marked increase in the infection-death ratio with age, countries with an older population structure are expected to have a higher percentage of deaths than countries with a relatively younger population structure,” said Nils Chr. Stenseth, Professor of Ecology and Evolution, University of Oslo. “Consistent with this, for example, South Africa – probably due in part to its younger population structure – has a lower death rate compared to older populations such as Italy. We found that such ‘death differences’ are strongly influenced by demographics. Regardless of demographics, however, we predict a consistent shift of risk to young people.”

The researchers said they designed their model to provide health authorities with a powerful and flexible tool to explore the future age circulation of COVID-19 for use in strengthening preparedness and deploying interventions.

Bjornstad said: “The mathematical framework we have built is flexible and can help align mitigation strategies for countries around the world with different demographic and social mixing patterns, making it a critical tool for policy decisions.”

Reference: “A General Model for the Demographics of the Transition from Pandemic Emergence to Endemism” by Ruiyun Li, C. Jessica E. Metcalf, Nils Chr. Stenseth and Ottar N. Bjørnstad, August 11, 2021, Science Advances.
DOI: 10.1126 / sciaadv.abf9040

The Huck Institute of the Life Sciences at Penn State and the Research Council of Norway supported this research.