Evolution from mild symptoms to severe COVID-19 in vaccinated and unvaccinated outpatients in Acapulco, Mexico: follow-up of a case series

María de J. Sosa-Martínez, Unidad de Medicina Familiar No. 9, Instituto Mexicano del Seguro Social, Acapulco, Guerrero, Mexico
Víctor M. Alvarado-Castro, Centro de Investigación de Enfermedades Tropicales, Universidad Autónoma de Guerrero, Acapulco, Guerrero, Mexico
José Legorreta-Soberanis, Centro de Investigación de Enfermedades Tropicales, Universidad Autónoma de Guerrero, Acapulco, Guerrero, Mexico
Belén M. Sánchez-Gervacio, Centro de Investigación de Enfermedades Tropicales, Universidad Autónoma de Guerrero, Acapulco, Guerrero, Mexico
Claudia E. Rios-Rivera, Centro de Investigación de Enfermedades Tropicales, Universidad Autónoma de Guerrero, Acapulco, Guerrero, Mexico
Guillermina Juanico-Morales, Órgano de Operación Administrativo Desconcentrado Estatal Guerrero, Instituto Mexicano del Seguro Social, Acapulco, Guerrero, Mexico
Alfredo J. Lagarza-Moreno, Unidad de Medicina Familiar No. 9, Instituto Mexicano del Seguro Social, Acapulco, Guerrero, Mexico
Janet Saldaña-Almazán, Centro de Investigación de Enfermedades Tropicales, Universidad Autónoma de Guerrero, Acapulco, Guerrero, Mexico
Neil Andersson, Centro de Investigación de Enfermedades Tropicales, Universidad Autónoma de Guerrero, Acapulco, Guerrero, Mexico; Department of Family Medicine, CIET-PRAM, McGill University, Montreal, Quebec, Canada

Background: Although Coronavirus disease 2019 (COVID-19) vaccine effectiveness and age-related risk of severe disease are well established, real-world evidence from early vaccination phases remains relevant to understand how health systems operated during periods of rapid epidemiological transition. Objective: Assess the progression from mild to severe COVID-19 among vaccinated and unvaccinated outpatients during the Delta wave in a public health setting in Mexico. Method: A follow-up case series of 3,488 adult outpatients with laboratory-confirmed severe acute respiratory syndrome coronavirus 2 infection and mild disease at diagnosis, treated between May and August 2021, was conducted. Severe COVID-19 was defined as clinical deterioration requiring hospitalization and was confirmed through electronic medical records. Bivariate and Mantel-Haenszel multivariate analyses were performed to identify factors independently associated with severe outcomes. Results: Severe COVID-19 affected 3.2% of those initially presenting with mild symptoms. Some 12.6% of those with severe illness had received the vaccine. Vaccination protected against severe COVID-19 (adjusted odds ratio [ORa] = 0.05, confidence interval [CI] 95% 0.03-0.08). We found an association between severe COVID-19 and age 60 years or older (ORa = 9.3, CI 95% 5.9-14.6), and with one or more comorbidities (ORa = 6.2, CI 95% 4.0-9.5). Conclusion: During the Delta wave, vaccination substantially reduced the likelihood of severe COVID-19 among outpatients, while older age and comorbidities markedly increased risk. These findings provide contextual evidence of outpatient risk stratification and monitoring strategies implemented during an early phase of the pandemic.

Keywords: COVID-19. SARS-CoV-2. SARS-CoV-2 vaccine. Distance follow-up.