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

Stringent nonpharmaceutical interventions (NPIs) such as lockdowns and border closures are not currently recommended for pandemic influenza control. New Zealand used these NPIs to eliminate coronavirus disease 2019 during its first wave. Using multiple surveillance systems, we observed a parallel and unprecedented reduction of influenza and other respiratory viral infections in 2020. This finding supports the use of these NPIs for controlling pandemic influenza and other severe respiratory viral threats.

1. I had a lot of trouble understanding the sentence: 'Influenza virus has a shorter serial interval and earlier peak infectivity compared to SARS-CoV-2. Our recent publication also showed that up to 32% of influenza virus infections in NZ are mild or asymptomatic, suggesting the likelihood of substantial asymptomatic transmission.6' I am not sure what the authors mean by 'serial interval' and the issue of substantial aysmtomatic transmission is also an issue with COVID-19 (80% have mild infections). Regarding the second point, are they suggesting that flu is less than COVID-19? This is not clear. 2. Discussion. I think the authors need to also talk about data from Australia. There are a couple of interesting references: the Yeoh paper in CID (https://academic.oup.com/cid/advancearticle/doi/10.1093/cid/ciaa1475/5912591), the Britton et al paper in The Lancet (https://www.thelancet.com/journals/lanchi/article/PIIS2352-4642(20)30307-2/fulltext) and the NSW surveillance reports (https://www.health.nsw.gov.au/Infectious/covid-19/Pages/weeklyreports.aspx) 3. Discussion: The Britton paper (Australia) provides a theory for why Rhinoviruses are less affected by the COVID-19 control measures: 'Rhinoviruses are easily transmitted between children in close contact and are non-enveloped so might be inherently less susceptible to inactivation by handwashing.' I think the authors should consider this point in their paper. 4. Discussion. The NSW surveillance reports indicate that RSV is increasing strongly at the end of the winter. It would be very interesting for the NZ group to comment about this. It doesn't look like NZ is seeing a similar pattern. Or is this because schools have been opened in NSW and not in NZ? 5. Discussion: The authors make the following statement: 'We postulate that NZ's use of stringent NPIs (lockdowns and border controls) have markedly changed human behaviour, resulting in substantial reductions in contacts between influenza-infected individuals and influenza-susceptible individuals.' I find it surprising that the research group can collate such detailed data on respiratory infections but not provide (some) behavioural data to support this statement. There must be some behavioural data for NZ to support this (e.g. the Google movement data). 6. Discussion. I miss a section about the role of children in the Discussion. If I understand rightly schools have been completely closed in NZ (full lockdown period). Has this continued? If not, have they seen an increase in some respiratory infections (e.g. RSV, like in NSW)? 7. Discussion: Congratulations with the paragraph about WHO's pandemic influenza intervention guidance. This is well written and super relevant. I was very happy to see this paragraph. 8. Discussion: One important issue regarding influenza is whether laboratories are totally focused on COVID-19 and they are not testing for influenza (and other respiratory viruses). A statement is made about this point in the study limitations: 'Secondly, during the COVID-19 laboratory response, some laboratories prioritised testing for COVID-19 and reduced testing for influenza and other respiratory viruses.' Considering this is a surveillance group with access to detailed data, I think this hypothesis could be tested. Why don't they compare the tested specimen numbers over time? Has the number of specimens tested for influenza changed massively compared to previous years?

Rebuttal to REVIEWER COMMENTS
Reviewer #1 (Remarks to the Author): In this paper, Huang et al demonstrate the impact of non-pharmaceutical interventions for COVID-19 on influenza (and other respiratory viruses) in NZ in the winter of 2020. These findings are important and serve as a basis for future recommendations for control of pandemic influenza. This is generally well written manuscript and the analysis are simple, straightforward sound. I have a few comments for the authors: 1. How can you conclusively say that the virus reached NZ on 28 Feb? The first case may have been identified on this but that is not same as when virus reached NZ.
Rebuttal: We would like to thank the reviewer for the comment. The reviewer is correct. We cannot know for sure the date that the virus first reached New Zealand. We changed lines 50&51 to: "The coronavirus disease 2019 (COVID-19), declared a pandemic by the World Health Organization (WHO) on 11 March 2020, was first identified in a person in New Zealand (NZ) on 28 February 2020."

This has also been seen in other southern hemisphere countries like Australia, Argentina, Columbia etc. so should be referenced
Rebuttal: We would like to thank the reviewer for the comment and added reference for other southern hemisphere countries. We changed lines 156&157: "NZ data, presented here, is consistent with what reported from other southern hemisphere countries 1,2 in Australia, Chile and South Africa, as well as reported from Hong Kong during the 2003 SARS epidemic, 3 and the COVID-19 pandemic. 4 Therefore, we suggest it is important to re-evaluate the role of stringent NPIs such as lockdowns and border closures in mitigating or even eliminating severe pandemic influenza. Although such measures are associated with significant negative impacts on society, their potential beneficial effects on delaying, containing or averting transmission and saving lives should be assessed. New knowledge from this assessment may inform better preparedness for future influenza pandemics and other severe respiratory viral threats. Additionally, it would be a worthwhile endeavour to conduct detailed analysis to identify which components of NPIs were most effective for preventing seasonal influenza and other respiratory virus infection and transmission. Careful investigation of NPIs may identify new and sustainable interventions that can minimize and prevent seasonal and epidemic respiratory viral illnesses in the future." Reviewer #2 (Remarks to the Author): General comment: This is a very nice paper. Congratulations. It would be nice to publish the paper asap as it is highly relevant to countries in the Northern Hemisphere. I have some suggestions to improve the text.

I had a lot of trouble understanding the sentence: 'Influenza virus has a shorter serial interval and earlier peak infectivity compared to SARS-CoV-2. Our recent publication also showed that up to 32% of influenza virus infections in NZ are mild or asymptomatic, suggesting the likelihood of substantial asymptomatic transmission.6' I am not sure what the authors mean by 'serial interval' and the issue of substantial aysmtomatic transmission is also an issue with COVID-19 (80% have mild infections). Regarding the second point, are they suggesting that flu is less than COVID-19? This is not clear.
Rebuttal: We would like to thank the reviewer for the comment. We amended lines 70&73: "Influenza virus has a short serial interval (the mean interval between illness onset in two successive patients in a chain of transmission) of 2-4 days. Viral excretion peaks early in the illness (i.e. during the first 1-3 days of illness). These features of influenza infection mean there is limited time to effectively implement isolation and quarantine measures. Additionally, substantial asymptomatic infection 8 creates difficulties in finding cases to initiate nonpharmaceutical measures. These characteristics have led to the assumption that these NPIs would not be effective in controlling influenza virus 9  Rebuttal: We would like to thank the reviewer for the comment and reference to the recent publication by Britton 10 et al that have reported the impact of COVID-19 public health measures on presentations to the Sydney Children's Hospitals Network with respiratory syncytial virus infections. We also cited the reference by Yeoh 2 et al that have reported respiratory syncytial virus and influenza detections in Western Australian children.
We have now cited both of these references in our reference to data reported from other southern hemisphere countries (see our response above to Reviewer 1, Question 2) and the role of handwashing in prevention of spread of non-enveloped viruses (see our response to Reviewer 2, Question 3).

Discussion: The Britton paper (Australia) provides a theory for why Rhinoviruses are less affected by the COVID-19 control measures: 'Rhinoviruses are easily transmitted between children in close contact and are non-enveloped so might be inherently less susceptible to inactivation by handwashing.' I think the authors should consider this point in their paper.
Rebuttal: We would like to thank the reviewer for directing us to this important recent reference. Hand washing results in the removal of dirt, organic material and transient microorganisms. During hand washing, friction is created and along with soap and water, this action removes soiling. Alcohol-based hand rubs have activity against non-enveloped viruses such as rhinovirus. Rhinovirus may be less susceptible to inactivation by soap-and-water type of hand washing. Additionally, children's generally poor quality of handwashing may also be another contributing factor. We amended lines 178&180: "Rhinovirus infections, responsible for more than one-half of cold-like illnesses, are frequently transmitted within households from children to other family members. 11 Additionally, rhinoviruses are non-enveloped viruses so might be inherently less susceptible to inactivation by soap-and-water handwashing. 10 Furthermore, the quality of children's handwashing is likely to be poor. These factors may have contributed to rhinovirus infection being less affected by the COVID-19 control measures."

Discussion. The NSW surveillance reports indicate that RSV is increasing strongly at the end of the winter. It would be very interesting for the NZ group to comment about this. It doesn't look like NZ is seeing a similar pattern. Or is this because schools have been opened in NSW and not in NZ?
Rebuttal: The reviewer is correct that we did not see RSV increase during the 2020 winter season in NZ. Schools have been fully open throughout NZ since 13 May 2020 (note: Auckland had a regional lockdown at Alert level 3 in August 2020 and schools may not function as normal as other regions.) It appears that school opening in late winter months in NZ was not associated with the increase of RSV activity at the end of the winter.

Discussion: The authors make the following statement: 'We postulate that NZ's use of stringent NPIs (lockdowns and border controls) have markedly changed human behaviour, resulting in substantial reductions in contacts between influenza-infected individuals and influenza-susceptible individuals.' I find it surprising that the research group can collate
such detailed data on respiratory infections but not provide (some) behavioural data to support this statement. There must be some behavioural data for NZ to support this (e.g. the Google movement data). Rebuttal: The evidence of behavioural change during NZ's lockdown has been documented in the supplementary figure 2 in the publication 12  We have cited this reference in this sentence in line 145: "We postulate that NZ's use of stringent NPIs (lockdowns and border controls) have markedly changed human behaviour 12 , resulting in substantial reductions in contacts between influenzainfected individuals and influenza-susceptible individuals."