13.09.2021

According to new model calculations, COVID-19 could shift from older adults to younger children if the SARS-CoV-2 virus becomes endemic and remains more widespread.

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In the next few years, when the SARS-CoV-2 virus becomes endemic in the world’s population, COVID-19 can behave like other common cold coronaviruses, affecting especially young children who have not yet been vaccinated or exposed to the virus . Because the severity of COVID-19 is generally lower in children, the overall burden of the disease is expected to decrease.

The burden of disease decreases with decreasing age

“When infected with SARS-CoV-2, there was a clear tendency towards increasingly serious consequences and deaths with increasing age,” explained Professor Ottar Björnstad, an author of the study, which was published in the journal Science Advances. “Our modeling suggests that the risk of infection is likely to shift to younger children if the adult population becomes immune through either vaccination or exposure to the virus.”

Similar examples in the past

Björnstad pointed out that such shifts have already been observed with other coronaviruses and influenza viruses. “Historical records of respiratory diseases indicate that the incidence of the disease in different age groups increases during an epidemic [zeitlich und regional begrenzter Krankheitsausbruch] could be very different from the endemic distribution, ”he clarified. For example, ongoing genomic work suggests that the 1889-1890 pandemic, sometimes known as Asian or Russian flu – which killed a million people, mostly adults over the age of 70 – may be due to the appearance of the HCoV-OC43 virus was caused. Over time, this has become an endemic, mild, repetitive cold virus that primarily affects children aged 7 to 12 months.

With rapidly dwindling immunity, recurring severe outbreaks would be possible

Bjornstad warned, however, that the burden of disease could remain high as immunity to reinfection from SARS-CoV-2 in adults wanes.

“Experience with seasonal coronavirus suggests that prior exposure may only confer short-term immunity to re-infection and that recurring outbreaks are possible. “Research on COVID-19 shows that vaccination offers greater protection than exposure to the SARS-CoV-2 virus, so we encourage everyone to get vaccinated as soon as possible.”

“Realistic age-structured” calculation model

The American-Norwegian team developed a so-called “realistic age-structured (RAS) mathematical model” that integrates demographics, the degree of social mixing and the duration of infection-blocking and disease-reducing immunity in order to investigate potential future scenarios.

Incidence and Mortality Burden for COVID-19

Specifically, the researchers examined the disease burden for 1, 10 and 20 years. They also analyzed how 11 different countries – including China, Japan, South Korea, Europe, Spain, Great Britain, France, Germany, Italy, USA, Brazil and South Africa – that differ greatly in their demographics, performed in these computational models.

“Regardless of immunity and mixing, the death rate can vary from country to country due to different age structures,” says Ruiyun Li, postdoctoral fellow at the University of Oslo. “Our general model framework enables robust predictions of age-related risk in the face of short- or long-term protective immunity, a reduction in disease severity with previous exposure, and taking into account the demographics of different countries […].”

According to Li, it’s well documented that social distancing affects transferability. Therefore, the team’s model assumes that the reproductive number (R0) – or the degree of portability – on a given day is linked to mobility on that day. The model also encompasses a variety of scenarios for immunity, including the dependence of disease severity on previous exposure, short-term (either three months or one year) and long-term (either 10 years or permanent) immunity.

Model probably only applies if renewed infection is milder

Princeton University Professor Jessica Metcalf found that this prediction is likely to be correct only if reinfections cause only mild illness. Mortality could therefore remain unchanged over time if primary infections do not prevent reinfections or alleviate severe illnesses in the elderly.

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

Different results for different countries show dependency on population structure

“In view of the significant increase in the infection mortality rate with age, it is to be expected that countries with an older population structure have a higher proportion of deaths than countries with a relatively young population structure,” explained Professor Nils Chr. Stenseth from the University of Oslo. “Consistent with this, South Africa, for example, has a lower number of deaths compared to countries with a higher proportion of older populations like Italy, probably due in part to its younger population structure. We found that such differences in death rates are heavily influenced by demographics. Regardless of the demographics, however, we predict that the risk will be consistently shifted to the young. “

Sources: EurekAlert! ScienceAwards

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