The new
coronavirus
is not an equal-opportunity killer: Being elderly and having other
illnesses, for instance, greatly increases the risk of dying from the
disease the virus causes, Covid-19. It’s also possible being male could
put you at increased risk.
For both medical and public health reasons, researchers want to
figure out who’s most at risk of being infected and who’s most at risk
of developing severe or even lethal illness. With that kind of
information, clinicians would know whom to treat more aggressively,
government officials would have a better idea of steps to take, and
everyone would know whether they need to take special, additional
precautions.
Here’s what research has shown three months into the outbreak:
Old and young
The vast majority of cases in China — 87% — were in people ages 30 to 79, the China Center for Disease Control
reported
last month based on data from all 72,314 of those diagnosed with
Covid-19 as of Feb. 11. That probably reflects something about biology
more than lifestyle, such as being in frequent contact with other
people. Teens and people in their 20s also encounter many others, at
school and work and on public transit, yet they don’t seem to be
contracting the disease at significant rates: Only 8.1% of cases were
20-somethings, 1.2% were teens, and 0.9% were 9 or younger. The World
Health Organization mission to China found that 78% of the cases
reported as of Feb. 20 were in people ages 30 to 69.
The death toll skews old even more strongly. Overall, China CDC
found, 2.3% of confirmed cases died. But the fatality rate was 14.8% in
people 80 or older, likely reflecting the presence of other diseases, a
weaker immune system, or simply worse overall health. By contrast, the
fatality rate was 1.3% in 50-somethings, 0.4% in 40-somethings, and 0.2%
in people 10 to 39.
The age-related death risk probably reflects the strength, or
weakness, of the respiratory system. About half of the 109 Covid-19
patients (ages 22 to 94) treated at Central Hospital of Wuhan,
researchers there
reported,
developed acute respiratory distress syndrome (ARDS), in which fluid
builds up in the small air sacs of the lungs. That restricts how much
air the lungs can take in, reducing the oxygen supply to vital organs,
sometimes fatally; half of the ARDS patients died, compared to 9% of
patients who did not develop the syndrome.
The ARDS patients had an average age of 61, compared to an average
age of 49 for those who did not develop ARDS. Elderly patients “were
more likely to develop ARDS,” the researchers wrote, suggesting how age
can make Covid-19 more severe and even fatal: age increases the risk
that the respiratory system will basically shut down under viral
assault.
Youth, in contrast, seems to be protective. The WHO mission reported a
relatively low incidence in people under 18, who made up only 2.4% of
all reported cases. In fact, through mid-January, zero children in
Wuhan, the epicenter of the outbreak, had contracted Covid-19. It’s not
clear whether that’s because children do not show signs of illness even
if infected.
Even cases among children and teens aged 10 to 19 are rare. As of
Feb. 11 there were 549 cases in that age group, 1.2% of the total, China
CDC found. Only one had died.
One intriguing explanation for the apparent resilience of youth: in
regions near Hubei province, young children seem especially likely to be
exposed to other
coronaviruses, scientists in China
reported in 2018. That might have given them at least partial immunity to this one.
Men and women
The effect of sex on susceptibility to Covid-19 is less clear than
the age effect, but preliminary data suggest men might be more
susceptible. China CDC found that 106 men had the disease for every 100
women, while the WHO mission found that men make up 51% of cases. A
study of 1,099 Covid-19 patients in Wuhan through Jan. 29 found a
greater imbalance: 58% were male, the China Medical Treatment Expert
Group for Covid-19
reported last week in the New England Journal of Medicine.
It’s possible the apparent sex imbalance reflects patterns of travel
and contacts that make men more likely to be exposed to carriers of the
virus, not any inherent biological differences. It’s also possible the
apparent worse disease severity in men could skew the data. Among
hospitalized patients, there is “a slight predominance of men,” U.S.
researchers
wrote
last week in the Journal of the American Association. If the virus hits
men harder than women, health care systems will see, test, and count
more men.
To determine if there are sex differences in susceptibility to
infection would require an unethical experiment: expose 1,000 otherwise
similar men and women to the virus and see who gets sick.
The difference is fatality rates, however, is real: 1.7% for women and 2.8% for men, China CDC reported.
Sick or healthy
The male-female difference in fatality rates, and perhaps in reported
incidence, may arise from differences in underlying health. People with
pre-existing illness are more likely to get seriously ill from
Covid-19, and men have a higher incidence of such chronic illnesses as
cardiovascular disease.
In the first large
study
of the effect of underlying illness, researchers in China analyzed
1,590 patients from throughout the country with laboratory-confirmed
disease. They calculated how “co-morbidities” — existing illnesses —
affected the risk of being admitted to intensive care, being put on a
ventilator, or dying.
After taking into account the patients’ ages and smoking status, the
researchers found that the 399 patients with at least one additional
disease (including cardiovascular diseases, diabetes, hepatitis B,
chronic obstructive pulmonary disease, chronic kidney diseases, and
cancer) had a 79% greater chance of requiring intensive care or a
respirator or both, or of dying, they reported last week in a paper
posted to medRxiv, a preprint site that posts research before it has
been peer-reviewed. The 130 with two or more additional diseases had 2.5
times the risk of any of those outcomes.
That fits with what’s known about other respiratory viruses. People
with a single co-morbidity who catch avian flu (H7N9) are 3.4 times more
likely than otherwise-healthy flu patients to require ventilators and
other intensive care. SARS and MERS, which are also caused by
coronaviruses, are more severe in patients with underlying illnesses,
too.
Breaking down the Covid-19 risk with the most common co-morbidities,
the scientists found that cancer raises the risk 3.5-fold, COPD
2.6-fold, and diabetes and hypertension by about 60%. Because it isn’t
unusual for someone to have an underlying disease (especially diabetes
and hypertension) but not know it, the last figure is likely an
underestimate.
Co-morbidities also raise the risk of dying from Covid-19. China
CDC’s analysis of 44,672 patients found that the fatality rate in
patients who reported no other health conditions was 0.9%. It was 10.5%
for those with cardiovascular disease, 7.3% for those with diabetes,
6.3% for people with chronic respiratory diseases such as COPD, 6.0% for
people with hypertension, and 5.6% for those with cancer.
Underlying disease might change the course of Covid-19. During the
height of the epidemic in Wuhan, 37 of 230 patients receiving dialysis
for kidney failure at Remnin Hospital developed the disease. Even though
none were sick enough to require intensive care or a mechanical
ventilator, six of them died, for a very high fatality rate of 16%.
Oddly, however, none of the six died of pneumonia, Remnin researchers
reported.
Instead, the causes of death were heart disease, stroke, and high blood
levels of potassium (a result of kidney failure). The high fatality
rate of Covid-19 in already-sick people might result not from the virus
but from an exacerbation of existing disease.
About 60% of U.S. adults have at least one underlying health
condition, Tom Frieden, former director of the U.S. Centers for Disease
Control and Prevention, told reporters on Monday.
Pregnancy
In early February, Chinese state media
reported
that a woman infected with the virus gave birth to a baby who later
tested positive for it. Newborns might become infected because of close
proximity to a patient, like anyone else, but the case raised fears that
a pregnant woman can transmit the virus to her fetus via the placenta.
Only one small study has investigated such “vertical transmission.” Scientists at Wuhan University
found
that, of nine pregnant patients infected with the virus (all had a
caesarean section) in their third trimester, none seemed to pass the
virus to their babies, all of whom scored at the top of the Apgar scale
of newborn health.
As for the mothers, “Covid-19 seems not to be especially severe in
pregnant women, at least based on the small number in this study,” the
scientists wrote. That was somewhat surprising because pregnancy
suppresses the immune system (so it doesn’t attack the fetus); pregnant
women are more susceptible to respiratory pathogens than non-pregnant
women. Nevertheless, none of the nine women developed severe Covid-19
pneumonia.
It may be that immuno-suppression is actually helpful. Some of the
most serious symptoms of Covid-19 result from an immune system on the
rampage rather than a lethargic one, Chinese scientists
found:
An extreme immune response called cytokine storm, a flood of immune
cells and the biochemicals they produce, tears through lung tissue.
Who is getting sick, and how sick? A breakdown of coronavirus risk by demographic factors
