Changes in the epidemiological profile of SARS-CoV-2-positive individuals in Mexico across pandemic waves as an explanation of fatality reduction: a retrospective observational study

OBJECTIVES: We aim to quantify shifts in hospitalisation and mortality and how those were related to the first three phases of the epidemic and individuals’ demographics and health profile among those with a positive test for SARS-CoV-2 treated at the Mexican Social Security Institute’s facilities from March 2020 to October 2021. DESIGN: Retrospective observational study using interrupted time series analysis to identify changes in hospitalisation rate and case fatality rate (CFR) by epidemic wave. SETTING: Data from the Mexican Institute of Social Security’s (IMSS) Online Influenza Epidemiological Surveillance System (SINOLAVE) that include all individuals that sought care at IMSS facilities all over Mexico. PARTICIPANTS: All individuals included in the SINOLAVE with a positive PCR or rapid test for SARS-CoV-2. PRIMARY AND SECONDARY OUTCOME MEASURES: Monthly test positivity rates, hospitalisation rates, CFRs and prevalence of relevant comorbidities by age group. RESULTS: From March 2020 to October 2021, the CFR declined between 1% and 3.5%; the declines were significant for those 0–9, 20–29, 30–39, 40–49 and 70 and older. The decline was steep during the first wave and was less steep or was temporarily reversed at the beginning of the second and third waves (changes in the trend of about 0.3% and 3.8%, and between 0.7% and 3.8%, respectively, for some age groups), but then continued to the end of the analytical period. Prevalence of diabetes, hypertension and obesity among patients testing positive also declined—two for most age groups (reductions of up to 10 percentage points for diabetes, 12 percentage points for hypertension and 19 percentage points for obesity). CONCLUSION: Data suggest that the decrease in COVID-19 fatality rate is at least partially explained by a change in the profile of those contracting the disease, that is, a falling proportion of individuals with comorbidities across all age groups.