Healthcare systems seem may be ill-equipped to handle high co-circulation of influenza and SARS-CoV-2 in the coming year

In a recent study posted to the medRxiv* pre-print server, researchers conducted a community-based prospective cohort study in Managua, Nicaragua, before the beginning of the winter season when influenza A (H3N2) cases surge in the Northern Hemisphere.

Background

Closely monitoring influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) co-circulation could help understand the cumulative burden on healthcare facilities faced with simultaneous respiratory epidemics. This data could help public health officials devise a strategy to counter high co-circulation of influenza and SARS-CoV-2 during the winter/fall season and ease the burden on the public healthcare system.

Influenza transmission markedly decreased globally during the first two years of the coronavirus disease 2019 (COVID-19) pandemic. For instance, Nicaragua had just five cases of influenza, of which 80% were caused by influenza B, in 2021. However, influenza cases started surging again in 2022,  with a substantial increase in influenza circulation in Nicaragua. It is a worrisome trend as the typical Northern Hemisphere influenza season is approaching.

About the study

In the present study, researchers examined influenza, SARS-CoV-2 infections, and coinfections between January 1 and July 20, 2022, in the participants of the Household Influenza Cohort Study held in Nicaragua. They collected respiratory samples from study participants who visited the clinic upon developing fever, conjunctivitis, rash, or loss of taste or smell.

Further, the researchers tested these samples for influenza using the Center of Disease Prevention and Control (CDC) protocols and SARS-CoV-2 by real-time reverse transcription-polymerase chain reaction (RT-PCR). Notably, they also collected samples from other household members of those participants who tested positive for either influenza or SARS-CoV-2.

Furthermore, the researchers calculated the incident rates of each pathogen using a Poisson distribution and compared the observed and expected coinfection cases using the chi-squared test. Finally, they calculated the attack rates of influenza and SARS-CoV-2 by dividing the number of cases of each pathogen by the total number of participants enrolled in the study.

Study findings

The study population comprised 2117 participants aged 0-89 years, of which 62.5% were females. Gender did not affect the incidence rates of both diseases. The researchers noted 433 influenza and 296 SARS-CoV-2 infections, with incidence rates of 37.6 and 26 per 100 person-years, 95% confidence interval (CI), respectively. Notably, the influenza incidence rates peaked in children aged five years or less and then decreased steadily.

Intriguingly, age-stratified SARS-CoV-2 incidence rates displayed a slight V-shaped trend. In addition, 174, 105, and 38 households experienced influenza, SARS-CoV-2, and both infections. The coinfected population did not require hospitalization, but most had fever compared to those with only COVID-19. Despite high levels of hybrid immunity in the study population, more SARS-CoV-2 cases were severe/moderate than influenza. Moreover, more SARS-CoV-2 infections presented with cough, myalgia, and arthralgia compared to influenza, although both initially began with fever and upper respiratory symptoms.

The authors noted influenza A and SARS-CoV-2 co-circulation for 22 of 29 study weeks. Influenza and SARS-CoV-2 attack rates were 20.1% and 13.6%, which remained strikingly comparable even when standardized to the age distribution of the United States (US). Post-standardization attack rates for influenza and SARS-CoV-2 specifically were 17.2% for influenza and 14.3% for SARS-CoV-2.

In two to 14-year-olds, the influenza attack rate was 26.8%, whereas it was 15.3% for SARS-CoV-2. Compared to prior influenza incidence rates, the 2022 incidence rate for influenza A was substantially higher at 28.6 per 100 person-years. Furthermore, the researchers observed nearly the expected number of symptomatic influenza and SARS-CoV-2 coinfections.

Conclusions

The study highlighted the dual burden of influenza A and SARS-CoV-2 within a community-based cohort of households in Managua, Nicaragua. The co-circulation persisted for a staggering 75.9% of the study duration. Moreover, there were nearly as many coinfection cases as would have surfaced if these pathogens circulated independently. Overall, the study findings depicted a substantial burden on the healthcare system.

It is worrisome that the US population is older than the study cohort; hence, similar levels of co-circulation would have led to more severe cases in the US. In addition, vaccination coverage remains low among those under 12 years. Considering the high attack rates of both viruses in children, this would result in substantial morbidity and further school disruptions. Considering the significant risk of dual epidemics of influenza and SARS-CoV-2, vaccine coverage for both influenza and SARS-CoV-2 is imperative before the coming influenza season.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.