Study population and setting
This study considered all probable and laboratory-confirmed cases of SARS-CoV-2 infection in New Zealand from February 2 (the date of a travel ban from mainland China) to May 13, 2020 (by which time, community transmission had stopped). Probable cases were defined as close contacts of confirmed cases with compatible symptoms. Imported cases were defined as those with international travel within 14 days before symptom onset, import-related cases were defined as those with an epidemiologic link to imported cases, and all others were defined as locally acquired. Five phases of national response were defined: Phase 1 (February 2 to March 15) comprised initial travel restrictions; Phase 2 (March 16 to 25) included rapid scale-up of non-pharmaceutical interventions including bans on public gatherings and closed borders except for returning residents; Phase 3 (March 26 to April 10) was the first half of lockdown and included widespread stay-at-home orders and expansion of test/trace/isolate protocols; Phase 4 (April 11 to 27) was the second half of lockdown and included testing expansion; and Phase 5 (April 28 to May 13) saw an easing of movement restrictions and permission for small gatherings. The authors related these response phases to case counts, disease outcomes, time-to-event durations, and transmission patterns. To assess outcomes related to transmission, cases were assigned to intervention phases on the basis of estimated infection date (one incubation period before case identification). To assess public health response, cases were assigned to intervention phases based on date of case notification. Logistic regression was used to identify risk factors for severe outcomes, and parametric distributions were fit to time-to-event durations.
Summary of Main Findings
There were 1,503 COVID-19 cases in New Zealand during the study period, of which 1,153 (77%) were confirmed, with 95 hospital admissions (6.3%) and 22 deaths (1.5%). Thirty-eight percent of cases were classified as imported, 31% were import-related, and 31% were locally acquired. The cumulative incidence nationwide was 303 per million, and the estimated infection rate per million peaked during Phase 2 (March 16 to 25) at 8.5. Females represented 56% of cases, and the highest cumulative incidence was seen in the age group of 20-34 years (464 per million). The share of cases identified via contact tracing more than doubled from Phase 1 (30%) to Phase 3 (66%). Testing incidence increased 220-fold from Phase 1 to Phase 5, and test positivity declined from a peak above 5% in late March to below 1% in the second week of April. Median time from symptom onset to case notification declined from 9.7 days in Phase 1 to 1.7 days in Phase 4. Median time from symptom onset to isolation declined from 7.2 days in Phase 1 to -2.7 days in Phase 4 (negative time implies cases were quarantined before symptom onset).
All COVID-19 cases in New Zealand during its epidemic were included, and authors described a broad range of epidemiologic parameters. The authors were able to draw from multiple comprehensive data sources for cases, testing, mobility, policy changes, demographic data, and other variables.
Though the aims of this study were more broad and the descriptive epidemiology presented is sound, few conclusions can be drawn from this study about the relative effectiveness of different NPIs implemented in New Zealand: each response phase encompassed multiple changes in NPIs, and the temporal relation of response phase to outcomes was imprecise. Moreover, the experience of New Zealand may not be particularly generalizable, given its status as a relatively small, economically developed island nation.
This paper provides a broad description of a rare success story during the COVID-19 pandemic: New Zealand’s rapid and efficient containment of its epidemic.
This review was posted on: 2 November 2020