Probability of symptoms and critical disease after SARS-CoV-2 infection

Abstract: We quantified the probability of developing symptoms (respiratory or fever \geq 37.5 {\deg}C) and critical disease (requiring intensive care or resulting in death) of SARS-CoV-2 positive subjects. 5,484 contacts of SARS-CoV-2 index cases detected in Lombardy, Italy were analyzed, and positive subjects were ascertained via nasal swabs and serological assays. 73.9% of all infected individuals aged less than 60 years did not develop symptoms (95% confidence interval: 71.8-75.9%). The risk of symptoms increased with age. 6.6% of infected subjects older than 60 years had critical disease, with males at significantly higher risk.

Probability of Symptoms and Critical Disease after SARS-CoV-2 Infection: An Analysis of Epidemiological Data from Lombardy, Italy

The paper undertakes a comprehensive epidemiological study focusing on the probability of developing symptoms and the risk of critical disease after SARS-CoV-2 infection, specifically within the context of Lombardy, Italy. Utilizing data from an extensive contact tracing effort conducted between February and April 2020, the authors examine 5,484 contacts of confirmed COVID-19 cases, identifying infections with SARS-CoV-2 through RT-PCR and serological assays.

Key Findings

1. Asymptomatic Infections: A significant proportion of individuals infected with SARS-CoV-2, particularly those under 60 years, remain asymptomatic. The study finds that 73.9% of infected individuals aged less than 60 did not exhibit symptoms, highlighting the prevalence of asymptomatic cases.

2. Age and Symptomatology: Age significantly influences the likelihood of symptom manifestation. For individuals aged 80 and above, 64.6% of infections resulted in symptomatic cases, whereas only 18.1% of infected subjects under 20 years developed symptoms.

3. Critical Disease Burden: The risk of developing critical disease, defined as requiring intensive care or resulting in death, is markedly higher in those over 60 years, with 6.6% experiencing severe outcomes compared to just 0.54% for those younger than 60. Male individuals are observed to have a heightened risk of critical disease.

4. Serological and PCR Testing: Notably, a significant portion of infections, 67.0%, were identified retrospectively via serological assays, pointing to the challenges of detecting infections promptly during epidemiological surveillance efforts.

Implications

The study provides insightful age-specific estimates of symptom development probability and critical disease risk following SARS-CoV-2 infection. These findings are pivotal for calibrating epidemiological models that forecast health system burdens and guide resource allocation during pandemic waves. The study gains additional relevance in its assessment of asymptomatic carriers, which complicates public health strategies centered on "test, trace, and isolate" due to the stealth transmission dynamics among symptom-free carriers, thus posing unique challenges in preventing local outbreaks.

Future Directions

The implications of these results call for continued advancements in epidemiological modeling of infectious diseases, with a particular focus on improving predictions of healthcare demands. The persistence of asymptomatic transmission warrants further research to accurately quantify its impact on community spread. Robust, age-stratified probability models of symptom development can aid in refining public health responses and enhancing preparedness for future pandemics, particularly in tailoring interventions and prioritizing vulnerable populations.

In conclusion, this paper contributes valuable data and insights pertinent to understanding the epidemiological characteristics of COVID-19 in Lombardy, thus offering lessons translatable to broader public health contexts and interventions.