- Several studies have investigated the risk the new coronavirus poses to specific groups
- A recent analysis explains why older people and men are more susceptible to severe Covid-19
- Differing immune systems may help explain why the risks vary so greatly
A new analysis looking into the mortality risk for Covid-19, the disease caused by SARS-CoV-2, shows that out of 1 000 people under the age of 50 who are infected with the virus, almost none will die.
However, for those in their fifties and early sixties, about five will die, with men at greater risk than women. And for every 1 000 people in their mid-seventies or older who are infected, around 116 will die.
The research team, whose analysis was published in preprint server medRxiv, and has not yet been peer-reviewed, looked at the presence of antibodies against SARS-CoV-2 in people in several countries, including England and Spain, following the first peak of the pandemic.
Antibody tests offer clues
The role of antibodies in protection against Covid-19 and the length of time survivors will be protected against reinfection have been studied for months.
Using data from antibody-prevalence studies, the team estimated mortality risk by age. In June and July, thousands of people across England received a pinprick antibody test in the post.
The team stated that of the 109 000 randomly selected teenagers and adults who took the test, around 6% harboured antibodies against the virus. This result was then used to calculate an overall infection fertility ratio (IFR) for England of 0.9% (or 9 deaths in every 1 000 cases).
According to existing studies, age is the strongest predictor of an infected person’s risk of dying – a metric known as IFR. This is the proportion of people infected with SARS-CoV-2 – including those who didn’t get tested or displayed symptoms – who will succumb to the disease.
For people between the ages of 15 and 44, the IFR was close to zero, and increased to 3.1% for those between the ages of 65 and 74, and to 11.6% for those older than 74.
'Age cannot explain everything'
However, “age cannot explain everything”, Henrik Salje, an infectious-disease epidemiologist at the University of Cambridge, UK, told Nature, and explained that gender is also a strong risk factor – with men almost twice as likely to die from Covid-19 than women.
Men face twice the risk
The researchers included a Spanish study from April, which also tested for antibodies in more than 60 000 residents in randomly selected households. A similar trend was observed in this study, where the overall IFR for the population was about 0.8%, but remained close to zero for people under 50.
As with the analysis above, this number increased swiftly to 11.6% for men 80 years old and over. However, it was 4.6% for women in that age group. According to the Spanish study, men are more likely to die of Covid-19 than women, and this risk increases with age.
“Men face twice the risk of women,” Beatriz Pérez-Gómez, an epidemiologist at the Carlos III Institute of Health in Madrid, who was involved in the Spanish study told Nature.
Why the difference in risk?
Jessica Metcalf, a demographer at Princeton University, New Jersey, explained to Nature that differences in the male and female immune-system response could explain the different risks: “The female immune system might have an edge by detecting pathogens just a bit earlier,” Metcalf said.
According to a recent study reported by Health24, growing evidence suggests that the viral load, antibodies and other immune responses may offer clues to why men are more susceptible to severe Covid-19 outcomes.
Among other findings, the research team of this study found that the females tended to produce more disease-fighting T-cells, leading to milder Covid-19 symptoms.
And in older male patients, T-cell levels appeared to decline even further – but not in female patients.
Metcalf further explained that older people are at higher risk of severe disease because, as the body ages, it develops low levels of inflammation, and Covid-19 could push an already overworked immune system over the edge.
Researchers of a study published in May also reported that the RNA that would usually attack a virus as soon as it tries to infect the body, diminishes with age and chronic health problems.