Impact of the COVID-19 nonpharmaceutical interventions on influenza and other respiratory viral infections in New Zealand

The COVID-19 pandemic, declared by the WHO on March 11, 2020, reached New Zealand (NZ) on February 28, 2020. NZ implemented progressive border restrictions and a four-level alert system from February 2, 2020, aiming for COVID-19 elimination. A Level-4 nationwide lockdown (March 25 - April 27, 2020) followed the emergence of community transmission. These stringent NPIs included border closure, widespread testing, case isolation, contact tracing, physical distancing measures (stay-at-home orders, school and business closures), individual infection prevention and control measures (hand hygiene, cough etiquette, mask wearing), and public risk communication. The effectiveness of NPIs depends on virus transmission characteristics. Influenza's short serial interval (2-4 days) and early peak viral excretion limit the impact of isolation measures. Significant asymptomatic infection further complicates control. The belief that NPIs are ineffective against influenza lacks robust field data. NZ's experience provides a natural experiment to assess the impact of NPIs on influenza and other respiratory viruses, informing pandemic influenza preparedness and seasonal planning.

The World Health Organization's (WHO) guidance on pandemic influenza interventions does not recommend stringent NPIs once sustained community transmission is established, based on the assumption of their ineffectiveness and impracticality. However, this guidance is based primarily on historical observations and modeling studies, lacking robust field data. Prior studies from other Southern Hemisphere countries (Australia, Chile, South Africa), Hong Kong during the 2003 SARS epidemic, and Hong Kong during the COVID-19 pandemic have shown similar reductions in respiratory illnesses under stringent NPIs, supporting the need for reevaluation of the WHO's current guidance.

This study utilized multiple national surveillance systems in NZ to assess the impact of NPIs on respiratory viral infections in 2020, comparing data to the 2015-2019 reference period. Data sources included: 1. **Hospital-based Severe Acute Respiratory Illness (SARI) surveillance:** Monitored SARI incidence rates and influenza-associated SARI in a population of ~1 million. 2. **Sentinel general practice (GP)-based surveillance:** Tracked influenza-like illness (ILI) incidence rates and influenza-associated ILI in ~10% of the NZ population. Note that this system's usual operation was altered in 2020 due to the COVID-19 response, potentially affecting data completeness. 3. **SHIVERS-II&III (Southern Hemisphere Influenza and Vaccine Effectiveness Research and Surveillance) community-based cohorts:** Monitored ILI and acute respiratory illness (ARI) incidence rates and influenza-associated cases in ~1400 adults and ~80 infants. 4. **National International Classification of Diseases (ICD)-coded influenza hospitalizations:** Recorded the number of influenza-related hospitalizations. 5. **Laboratory-based surveillance:** Tested samples for influenza and other respiratory viruses from various surveillance systems. Prioritization of SARS-CoV-2 testing in 2020 might have impacted data.

Multiple surveillance systems confirmed extremely low influenza activity in NZ during the winter of 2020. Hospital-based SARI surveillance showed very low incidence rates, with no influenza-associated SARI detected. Sentinel GP-based ILI surveillance, while affected by altered workflows, also reported low ILI rates and no influenza-associated ILI. SHIVERS-II&III showed lower ILI rates than in previous years. ICD-coded influenza hospitalizations declined significantly (p<0.001), and laboratory-based surveillance detected a marked reduction in influenza virus detections (p<0.001). Furthermore, substantial reductions were observed for other respiratory viruses (RSV, hMPV, enterovirus, adenovirus, PIV1-3, rhinovirus) during the post-lockdown period compared to the 2015-2019 reference period. However, a significant increase in rhinovirus was observed after restrictions eased to Level 1.

The near-elimination of influenza in NZ during the 2020 winter strongly suggests the effectiveness of stringent NPIs. The nationwide lockdown, implemented before the usual influenza season, combined with ongoing border controls, hygiene promotion, and social distancing measures, likely suppressed influenza transmission. The findings challenge the WHO's current guidance on pandemic influenza control, suggesting that stringent NPIs warrant reevaluation, considering their potential benefits against the societal costs. While other factors like vaccination rates, warmer winter temperatures, and viral-viral interactions might have contributed to the reduced influenza activity, the impact of NPIs appears to be the dominant factor. The increase in rhinovirus post-lockdown may be explained by its non-enveloped nature and potentially lower susceptibility to handwashing.

This study demonstrated the unprecedented reduction in influenza and other respiratory viral infections in New Zealand during 2020, largely attributable to stringent NPIs. These findings necessitate a reassessment of the WHO’s current pandemic influenza control guidelines. Future research should focus on identifying the most effective components of NPIs to inform future pandemic preparedness strategies and develop sustainable interventions for minimizing respiratory viral illnesses.

This observational study's limitations include the simultaneous implementation of multiple NPIs, making it difficult to isolate individual effects. The prioritization of SARS-CoV-2 testing in 2020 and altered workflows for sentinel GP-based ILI surveillance might have introduced selection bias and affected the completeness of the data. The study's findings are specific to the context of New Zealand and might not be generalizable to other settings with differing demographics, healthcare systems, and public health responses.