Director, Lead Epidemiologist Moderna, Inc., United States
Background: Co-circulation of SARS-CoV-2 during typical influenza seasons suggests the possibility of simultaneous infection, or coinfection, with 2 or more respiratory pathogens. Given these pathogens’ shared transmission dynamics, populations at risk for severe illness, and clinical presentation, understanding the epidemiology of coinfection can inform clinical decision-making and vaccine development.
Objectives: To review the contemporary literature for epidemiological evidence on influenza and SARS-CoV-2 coinfection.
Methods: An extensive literature search using PubMed, Web of Science, and Scopus was conducted between November 2022 and August 2023. Titles and abstracts were reviewed for relevance and classified before data was extracted. Text review focused on study design and setting, population, coinfection prevalence, risk factors, clinical symptoms and outcomes, as well as any findings related to the impact of vaccination on coinfection risk.
Results: A total of 29 adult/all-ages studies and 4 pediatric studies were reviewed (sample size of the studies ranged between 48 and 1.65 million), along with 11 systematic reviews/meta-analyses. Among the primary populations studied (those with either SARS-CoV-2 or influenza infection), coinfection rates were low ( < 1.0%). Studies with high coinfection rates generally had smaller sample sizes. While many studies evaluated a possible association between patient characteristics and coinfection risk, significant associations were not consistently demonstrated, except by 2 studies showing a potential association between older age (>50 or >60, respectively) and higher risk of coinfection. Coinfected patients had generally higher rates of respiratory symptoms and mortality compared with patients infected with only SARS-CoV-2 or influenza (5.8% compared to 0.8% and 3.0%, respectively). Coinfection was also associated with greater likelihood of admission to the intensive care unit, need for mechanical ventilation, length of stay, and cost. One study showed reduced coinfection risk among influenza vaccine recipients, and another found nonimmunized coinfected persons may experience more severe inflammatory response.
Conclusions: Recent evidence on various aspects of influenza and SARS-CoV-2 coinfection has important implications for future efforts aimed at prevention, risk reduction, and clinical management. Further research on influenza and/or SARS-CoV-2 vaccine impact on coinfection can benefit vaccine development and evaluation.
