30 June 2021
- We use a mathematical model to estimate the effect of New Zealand’s vaccine rollout on the potential spread and health impacts of COVID-19 and the implications for controlling border-related outbreaks.
- The model can be used to estimate the theoretical population immunity threshold, which represents a point in the vaccination rollout at which we could relax border restrictions with few or no controls in place and see only small occasional outbreaks.
- While there are significant uncertainties in R0 for new variants, for a variant that would have R0=4.5 with no public health measures (e.g. the Alpha variant), the population immunity threshold is estimated to require 83% of the population to be vaccinated under baseline vaccine effectiveness assumptions. For a variant with R0=6.0 (e.g. the Delta variant), this would need to be 97%.
- While coverage is below this threshold, relaxing controls completely would risk serious health impacts, including thousands of fatalities.
- Whether or not New Zealand reaches a theoretical population immunity threshold, the higher vaccination coverage is, the more collective protection the population has against adverse health outcomes from COVID-19, and the easier it will become to control outbreaks.
- Reaching or getting as close as possible to the population immunity threshold is very likely to require vaccinating at least some under-16-year-olds, subject to official approval for the vaccine to be used in these age groups.
- There remains considerable uncertainty in model outputs, in part because of the potential for the evolution of new variants. If new variants arise that are more transmissible or vaccine resistant, an increase in vaccine coverage will be needed to provide the same level of protection.
- A second important source of uncertainty arises because not all parts of the population will have equal vaccine coverage. Even if population immunity is achieved at a national level, communities with relatively low vaccine coverage or high contact rates will remain vulnerable to major outbreaks. These thresholds may also vary seasonally.
- Until the vaccine rollout is complete, retaining the elimination strategy will protect people who have not yet been vaccinated and, by keeping cases to a minimum, decrease the likelihood that the alert level system will be needed to control future outbreaks.
We present two implementations of an age-structured model for COVID-19 spread in Aotearoa New Zealand with a partially vaccinated population. The first is a deterministic SEIR model, useful for considering population-level dynamics and questions about population immunity. The second is a stochastic branching process, useful for considering smaller community outbreaks seeded by individual border arrivals. This builds on an earlier model used to inform the response to outbreaks of COVID-19 in New Zealand. The main purpose of this paper is to develop a model that can be used as the basis for policy advice on border restrictions and control measures in response to outbreaks that may occur during the vaccination roll-out. We consider a range of scenarios at different stages in the vaccine roll-out, including an unmitigated epidemic and contained local outbreaks. This work is intended to form a foundation for further COVID-19 vaccination modelling in New Zealand that will account for additional demographic variables.