Measuring the severity of COVID-19 using a model-based analysis

By Dr. Ananya Mandal, MDApr 1 2020

Researchers have modeled the progression and severity of COVID-19. Their study titled “Estimates of the severity of coronavirus disease 2019: a model-based analysis” was published in the latest issue of The Lancet Infectious Diseases.

The Pandemic

The world is now in the grips of the COVID-19 pandemic, with over 932,000 people infected, and over 46,000 people succumbing to the disease since December 2019. The case-fatality is conventionally expressed as a percentage and represents a measure of disease severity. Different studies have presented varying estimates, and this model-based analysis aimed to provide more accurate estimates of the case-fatality ratio to quantify the severity of COVID-19.

Different rates

“Studies done solely in hospitalized patients report the highest fatality ratios (8–28%), representing the outcome for the most severely ill patients. Estimates of the population-level case fatality ratio from all case reports are in the range of 2–8%. Estimates of the infection fatality ratio averaged across all age-groups range from 0·2% to 1·6%, while estimates of the infection fatality ratio in the oldest age group (≥80 years) range from 8% to 36%.”, say the researchers.

To obtain these numbers, the researchers looked at all published evidence up to 6^th March 2020. The papers were available at “PubMed, medRxiv, bioRxiv, arXiv, SSRN, Research Square, Virological, and Wellcome Open Research” and were all peer-reviewed articles, research reports, and pre-prints. Keywords SARS-CoV-2, COVID-19, and Coronavirus yielded these results, wrote the researchers.

Coronavirus COVID-19 quarantine. Image Credit: Mongkolchon Akesin / Shutterstock

The study

For this study, the team of researchers gathered case data from all the individuals who died due to COVID-19 with confirmed infection with SARS-CoV-2 in Hubei, Mainland China. The deaths were recorded up until 8^th February 2020. For the cases outside of China, data was gathered up until 25^th February from 37 nations, via government and health ministry websites and media reports.

For every individual case, the researchers looked at the following factors;

• The time between the onset of symptoms and outcome could either be death or hospital discharge.
• Case-fatality ratio for every age group.
• Age and location-based data.
• Case-based data from 1,334 cases detected outside mainland China.
• Data from the PCR confirmed cases to see the baseline prevalence of the infection among the international residents who had been repatriated from China. From these patients, as well as age-based case-fatality ratios were gathered.
• The proportion of patients needing hospitalization among 3,665 patients in China was estimated, and age-based case-fatality ratios were calculated.

What was found?

The outline of the results:

• 24 deaths occurred in Mainland China.
• Outside of China, there were 165 recoveries.
• The mean duration between onset of symptoms and death was 17.8 days (ranging between 16.9 and 19.2 days).
• The mean duration between onset of symptoms and hospital discharge was 24.7 days (ranging between 22.9 and 28.1 days).
• The crude case-fatality ratio among all cases from mainland China (a total of 70,117 cases) was 3.67 percent.
• A higher ratio was noted for the older age group in China. It was 0.32 percent in those aged less than 60 years and 13.4 percent in those aged over 80 years.
• A higher ratio was noted for the older age group among international cases. It was 1.4 percent in those aged less than 60 years and 4.5 percent in those aged over 60 years.
• After adjusting for other demographic parameters, the case fatality ratio within China was 1.38 percent.
• The infection fatality ratio in China was 0.66 percent, and this rose with age.
• The proportion of infected persons who are likely to be hospitalized rose with a maximum of 18.4 percent in persons over the age of 80 years.

Conclusions

Authors and have concluded that these “early estimates give an indication of the fatality ratio across the spectrum of COVID-19 disease and show a strong age gradient in risk of death.”

Commenting on the implications of their study, the team wrote that their estimates of the case fatality ratio of COVID-19 were less than the crude estimates found in other studies but higher for influenza pandemics (such as the H1N1 pandemic in 2009). They wrote, “Our estimate of the proportion of infected individuals requiring hospitalization, when combined with likely infection attack rates (around 50–80%), shows that even the most advanced health-care systems are likely to be overwhelmed. These estimates are, therefore, crucial to enable countries around the world to best prepare as the global pandemic continues to unfold.”

The study was funded by the UK Medical Research Council.