The leading cause of death is respiratory failure from acute respiratory distress syndrome (ARDS). The overall pooled mortality rate from ARDS in COVID-19 patients is 39%; however, this varies significantly between countries (e.g., China 69%, Iran 28%, France 19%, Germany 13%).
People <65 years of age have a very small risk of death even in pandemic epicenters, and deaths in people <65 years of age without any underlying conditions is rare.
Infection fatality rate (IFR)
Defined as the proportion of deaths among all infected individuals including confirmed cases, undiagnosed cases (e.g., asymptomatic or mildly symptomatic cases), and unreported cases. The IFR gives a more accurate picture of the lethality of a disease compared with the case fatality rate.
Approximately 10% of the global population may have been infected by October 2020, with an estimated overall IFR of 0.15% to 0.2% (0.03% to 0.04% in those <70 years of age).
The US Centers for Disease Control and Prevention’s current best estimate of the IFR, according to age (as of 10 September 2020):
0 to 19 years – 0.003%
20 to 49 years – 0.02%
50 to 69 years – 0.5%
≥70 years – 5.4%.
Based on these figures, the overall IFR for people <70 years of age is approximately 0.18%.
The IFR can vary across locations. A meta-analysis reports the point estimate of the IFR to be 0.68% across populations, with high heterogeneity (as of July 2020). The rate varied across locations from 0.17% to 1.7%.
Among people on board the Diamond Princess cruise ship, a unique situation where an accurate assessment of the IFR in a quarantined population can be made, the IFR was 0.85%. However, all deaths occurred in patients >70 years of age, and the rate in a younger, healthier population would be much lower.
These estimates have limitations and are likely to change as more data emerge over the course of the pandemic.
Estimates of the IFR can be inferred from seroprevalence studies.
UK: seroprevalence was 7.1% in the UK overall according to the first round of results of the UK Biobank COVID-19 antibody study. Previous infection was most common among people who lived in London (10.4%), and least common among those who lived in the south west of England and Scotland (4.4% in both).
US: seroprevalence estimates for 10 sites in the US are available. In the New York City metro area the number of estimated infections is at least 6 times higher than the number of cases reported according to the latest round of samples (7 to 11 July 2020). CDC: commercial laboratory seroprevalence survey data external link opens in a new window
China: seroprevalence was 3.2% to 3.8% in Wuhan, and decreased in other Chinese cities as the distance to the epicentre increased.
These studies suggest that the prevalence of infections is much higher than the official case counts suggest, and therefore the virus is much less lethal than initially thought.
Case fatality rate (CFR)
Defined as the total number of deaths reported divided by the total number of detected cases reported. CFR is subject to selection bias as more severe/hospitalised cases are likely to be tested.
The World Health Organization’s current estimate of the global CFR is 2.8% (as of 12 October 2020). This is much lower than the reported CFR of severe acute respiratory syndrome coronavirus (SARS), which was 10%, and Middle East respiratory syndrome (MERS), which was 37%.
CFR varies considerably between countries.
CFR increases with age.
In the US, the majority of deaths were in patients ages ≥65 years. The CFR was highest among patients ages ≥85 years (10% to 27%), followed by those ages 65 to 84 years (3% to 11%), then those ages 55 to 64 years (1% to 3%), and finally those ages 20 to 54 years (<1%).
In China, the majority of deaths were in patients ages ≥60 years. The CFR was highest among patients ages ≥80 years (13.4%), followed by those ages 60 to 79 years (6.4%), and then those ages <60 years (0.32%).
In Italy, the CFR was highest among patients ages ≥80 years (52.5%), followed by those ages 70 to 79 years (35.5%), and then those ages 60 to 69 years (8.5%).
CFR increases with the presence of comorbidities.
In China, the majority of deaths were in patients who had preexisting underlying health conditions (10.5% for cardiovascular disease, 7.3% for diabetes, 6.3% for chronic respiratory disease, 6% for hypertension, and 5.6% for cancer).
CFR increases with disease severity.
Limitations of IFR/CFR
Estimating the IFR and CFR in the early stages of a pandemic is subject to considerable uncertainties and estimates are likely to change as more data emerges. Rates tend to be high at the start of a pandemic and then trend downwards as more data becomes available.
There is currently no set case definition of a confirmed case, and case definitions vary. A positive polymerase chain reaction (PCR) result is sometimes the only criterion for a case to be recognized; however, a positive PCR test does not necessarily equal a diagnosis of COVID-19, or mean that a person is infected or infectious.
The number of deaths reported on a particular day may not accurately reflect the number of deaths from the previous day due to delays associated with reporting deaths. This makes it difficult to know whether deaths are falling over time in the short term.
Patients who die "with" COVID-19 and patients who die "from" COVID-19 may be counted towards the death toll in some countries. For example, in Italy only 12% of death certificates reported direct causality from COVID-19, while 88% of patients who died had at least one comorbidity.
Prognostic factors that have been associated with increased risk of unfavorable outcomes and mortality include:
Age ≥50 years
Presence of comorbidities (e.g., hypertension, diabetes, cardiovascular or cerebrovascular disease, COPD, obesity, malignancy)
Liver, kidney impairment, or cardiac injury
Elevated inflammatory markers (C-reactive protein, procalcitonin, ferritin)
A ratio of arterial partial pressure of oxygen to fraction of inspired oxygen (PaO₂/FiO₂) ≤200 mmHg and respiratory failure at admission are also independently associated with an increased risk of in-hospital mortality.
There is limited information about reinfection. Recurrent RT-PCR positivity in patients 1 to 60 days after recovery ranges between 7% to 23% in studies, with an estimated pooled rate of 12%. It is currently unclear whether this is due to reinfection, persistent viral shedding, or whether the test result was a false-negative at the time of discharge.
Studies have repeatedly reported positive RT-PCR tests for up to 90 days after initial infection; therefore, it is most likely that these cases are actually protracted initial infections. It is important to note that although persistent viral shedding has been reported for up to 90 days after the onset of infection, replication-competent virus has not been identified 10 to 20 days after the onset of symptoms (depending on disease severity).
True cases of reinfection (defined as two episodes of infection at least 3 months apart by virus strains with different genomic sequences) have been reported in Hong Kong, India, Ecuador, and Belgium. Two possible cases of reinfection have also been reported in the US; however, while different genomic variants were responsible for the two episodes in both men, the infections occurred less than 2 months apart.
The immune response, including duration of immunity, is not yet fully understood. However, there is limited evidence that suggests that infection with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is likely to confer some type of protective immunity against reinfection.
The immune response to SARS-CoV-2 involves both cell-mediated immunity and antibody production. Adaptive immunity to SARS-CoV-2 is thought to occur within the first 7 to 10 days of infection. A robust memory B-cell and plasmablast response is detected early in infection, with secretion of immunoglobulin A (IgA) and IgM antibodies by day 5 to 7, and IgG by day 7 to 10 from the onset of symptoms. IgA and IgM titers decline after approximately 28 days, and IgG titers peak at approximately 49 days. T-cells are simultaneously activated in the first week of infection and SARS-CoV-2-specific memory CD4+ and CD8+ T-cells peak within 2 weeks, but remain detectable for ≥100 days. Antibody and T-cell responses differ among individuals, and depend on disease severity.
While there have been concerns about early declining IgG neutralizing antibodies during convalescence, this is not thought to be an issue, because antibody levels always decline after the acute phase of an infection, and it is the levels of antibody titers after an infection that is important as this represents the generation of long-lived plasma cells to protect against subsequent infection.
Analysis of a large cohort of convalescent serum donors in New York City suggests that 99.5% of patients with confirmed mild disease seroconvert 4 weeks after illness. IgG antibodies developed over a period of 7 to 50 days from symptom onset, and 5 to 49 days from symptom resolution. This suggests that people with mild disease may have the ability to develop immunity. However, among patients who recovered from mild disease in China, neutralizing antibody titers varied substantially. There are data to suggest that asymptomatic people may have a weaker immune response to infection; however, this is yet to be confirmed.
Testing of blood samples taken before the COVID-19 pandemic have shown that some people already have immune cells that recognize SARS-CoV-2. Studies have reported T-cell reactivity against SARS-CoV-2 in 20% to 50% of people with no known exposure to the virus. This may be a consequence of true immune memory derived in part from previous infection with common cold coronaviruses, or from other unknown animal coronaviruses. However, further research into whether there is preexisting immunity to SARS-CoV-2 in the human population is required.
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