The reduction in vaccine effectiveness against COVID-19 transmission increases the risk. As such, carrying out a frequent routine testing of highly exposed vaccinated groups could mitigate the increased risk due to vaccination
Professor Michael Plank, Dr Rachelle Binny, Professor Shaun Hendy, Dr Audrey Lustig, Dr Kannan Ridings


Throughout 2020 and the first part of 2021, Australia and Aotearoa New Zealand have followed a COVID-19 elimination strategy. Both countries require overseas arrivals to quarantine in government-managed facilities at the border. In both countries, community outbreaks of COVID-19 have been started via infection of a border worker. This workforce is rightly being prioritized for vaccination. However, although vaccines are highly effective in preventing disease, their effectiveness in preventing infection with and transmission of SARS-CoV-2 is less certain. There is a danger that vaccination could prevent symptoms of COVID-19 but not prevent transmission.


We use a stochastic model of SARS-CoV-2 transmission and testing to investigate the effect that vaccination of border workers has on the risk of an outbreak in an unvaccinated community. We simulate the model starting with a single infected border worker and measure the number of people who are infected before the first case is detected by testing.


We show that if a vaccine reduces transmission by 50%, vaccination of border workers increases the risk of a major outbreak from around 7% per seed case to around 9% per seed case. The lower the vaccine effectiveness against transmission, the higher the risk.


The increase in risk as a result of vaccination can be mitigated by increasing the frequency of routine testing for high-exposure vaccinated groups.

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First published: Sep 29, 2021
COVID-19 Modelling Aotearoa
[COVID-19 Modelling Aotearoa](, is a cross-organisation and transdisciplinary group of academic researchers and scientists that were brought together by Te Pūnaha Matatini to help Aotearoa New Zealand face the COVID-19 pandemic. Our work is underpinned by Te Tiriti o Waitangi, fast and committed peer review, and ethics. These parameters ensure that the modelling developed by our broad team is uniquely equipped to provide scientifically robust results which are fit for Aotearoa New Zealand and support our decision-making.