The Return of Wintertime Respiratory Virus Outbreaks and Shifts in the Age Structure of Incidence in the Southern Hemisphere

To the Editor—Nonpharmaceutical interventions (NPIs) implemented to mitigate the transmission of severe acute respiratory syndrome coronavirus 2 also disrupted the transmission of endemic seasonal respiratory viruses. Influenza activity, for example, has been historically low in both the Northern and Southern Hemispheres for the past 2 years [1]. However, with the easing of physical distancing policies and the build-up of susceptible individuals in the population, the possible trajectory of respiratory viruses this year remains unknown and may vary by pathogen type [2]. Understanding the timing, intensity, and age structure of wintertime outbreaks in the Southern Hemisphere provides clues for the possible trajectory in the upcoming Northern Hemisphere winter.

Here, we examined a uniquely detailed dataset of laboratory-confirmed cases from the Global Influenza Hospital Surveillance Network in Chile, as well as countrywide diagnoses from emergency room (ER) visits (Figure 1). Consistent with observations from other countries [3, 4], these analyses show that the incidence of respiratory viruses declined abruptly after the introduction of NPIs and was almost absent during 2020 and 2021 but has resurged in 2022. Viruses that infect mostly infants and young children had an atypical and much reduced season in 2021, with respiratory syncytial virus (RSV) and parainfluenza peaking 14 and 25 weeks later than in 2019, respectively. Metapneumovirus, adenovirus, influenza A, and influenza B did not experience an outbreak in 2021, but influenza A experienced a small summer peak in January 2022, which is unexpected for this region, whereas influenza B has essentially disappeared since May of 2020, a trend that has been reported in several places [1, 4]. The country has already experienced the winter 2022 respiratory illness season with RSV, parainfluenza, metapneumovirus, adenovirus, and influenza A back to their typical seasonal timing. This return to prepandemic seasonality is reflected in both the surveillance and ER visit data (Figure 1A–C), suggesting that these results are unlikely to be an artifact of any changes in the surveillance system. While the maximum of total laboratory-confirmed cases from this year exceeds 1800 (compared to 1300 in 2019, Figure 1B) due to a greater number of tests conducted (780 more tests at the peak in 2022 compared to 2019), the number of national ER visits diagnosed as acute lower respiratory infections and influenza have not surpassed the values of previous years (Figure 1C). This suggests that while respiratory viral infections have returned, the outbreaks have not been more severe during the 2022 season.

Figure 1.

Respiratory illness surveillance data from the Global Influenza Hospital Surveillance Network in Chile. A, The percentage positive out of the total tested is shown for each virus. Data from https://www.ispch.cl/virusrespiratorios/. B, Number of laboratory-confirmed cases by pathogen (left y-axis) and number of tests performed each week (right y-axis, gray line). C, Total national emergency room visits coded as International Classification of Diseases, Tenth Revision J20–J21 (acute lower respiratory infections) and J09–J11 (influenza). Data from https://deis.minsal.cl/. D, Laboratory-confirmed cases during the winter season (20 June–23 September) of 2018, 2019, and 2022 by pathogen and age group. Abbreviations: ER, emergency room; RSV, respiratory syncytial virus.

Strikingly, the resumption of respiratory viral circulation during the last 2 years has been accompanied by an increase in incidence among older children, an outcome that has been predicted theoretically [5, 6] but has not been shown with empirical data elsewhere. The predicted increase in incidence among older children is due to the build-up of susceptible individuals who were less likely to get infected earlier in childhood due to very low circulation of the disease in the past 2 years. We show that children between 1 and 4 years of age dominated the outbreaks of RSV (51%) and parainfluenza cases (56%) in 2021, surpassing the values of infants <1 year of age (32% and 23%, respectively). We observed a similar trend during the winter of 2022, where 43% of RSV cases, 27% of parainfluenza cases, 27% of metapneumovirus cases, and 21% of adenovirus cases were in infants <1 year of age, while children between 1 and 4 years of age made up 34%, 43%, 42%, and 63% of cases, respectively. This trend is contrary to that observed prepandemic when the proportion of cases in the youngest group was always the highest (Figure 1D). We note that cases in those who are 5 to 14 years old have also experienced a significant increase in the test-positive percentage, compared to previous years. ER visits due to acute lower respiratory infections show an increase in older children, too (Figure 1C).

We acknowledge the limitations of relying on International Classification of Diseases, Tenth Revision coding data, which may be biased due to misclassification of diseases with similar symptoms. However, the strong correspondence between our 2 data sources suggests that these biases are likely to be small, and the ER data reflect the overall time trend in respiratory diseases. In summary, we show that while the incidence of respiratory infections in the Southern Hemisphere has returned to typical seasonal timing for childhood diseases, the relative burden has shifted toward older children, likely driven by an increase of susceptible individuals in the older groups. Given the synergies in seasonal respiratory diseases between the Southern and Northern Hemisphere, our findings have important implications for what the Northern Hemisphere might expect during the upcoming winter season. Our results also highlight the importance of improving influenza vaccine uptake; this may be especially important for reducing the burden on the healthcare system more generally during the winter months.

Notes

All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.