The latest research from Te Pūnaha Matatini shows that if vaccination rates aren't high enough, New Zealand will continue to need lockdowns or see thousands of Covid-19 deaths a year, Marc Daalder reports
High vaccination rates are crucial to ending the use of lockdowns, expert disease modellers have found.
Te Pūnaha Matatini researchers Nicholas Steyn, Michael Plank and Shaun Hendy have modelled the outcome of a year-long outbreak of the Delta variant at a range of different vaccination thresholds and with different levels of public health restrictions in place.
If vaccines remain effective against the virus or are reinforced with booster shots, if more than 90 percent of the population over the age of five is vaccinated and if moderate public health measures (like mandatory masking and improved ventilation) are put in place alongside a comprehensive testing, tracing and case isolation system, then the impact of the virus could be similar to an average flu year. If vaccines are not available for the five-11 age cohort, then more than 95 percent of the eligible over 12 population would have to be vaccinated.
Even vaccination rates considered high overseas – like 80 percent of over 12s vaccinated – would see 7000 New Zealanders die over the course of a year and 5770 people in hospital at the peak of the outbreak.
The results reiterate the findings of Te Pūnaha Matatini modelling from June, which found vaccination alone was unlikely to get New Zealand to population immunity. But they go further in confirming that active management of outbreaks will still be needed to stop the health system from being overwhelmed.
The key to stopping an outbreak is to reduce the reproduction number (how many people each case infects) to below 1. Hopes that vaccines alone can reduce the R number below 1 have dwindled with the arrival of the more transmissible Delta variant, which has a natural R number of 5.5 to 6.5.
Masking and other widespread but less intrusive public health measures will help depress the R number and vaccines will still make a significant difference, reducing transmission by around 85 percent. But active testing for new cases, contact tracing of positive results, isolation of contacts and quarantine of Covid-19 cases will still be needed, the research has found.
"In order to keep annual mortality from Covid-19 at levels similar to or below those experienced from seasonal influenza, for instance, the effective reproduction number [...] of the virus must be kept close to or below 1 by a combination of vaccination, targeted measures such as case isolation, and other control measures," the study found.
"Results from both Te Pūnaha Matatini’s SEIR vaccination model and the ESR model suggest that if the reproduction number is even slightly greater than 1, fatalities would be measured in thousands, despite high vaccine coverage."
The researchers said it would be important for children to be vaccinated if the Pfizer vaccine is found to be safe and effective in the five-11 age group. While 90 percent vaccination of over 12s and effective testing, tracing and isolation would limit the damage of an outbreak, it would still lead to 461,000 cases. Almost a third of these would be in children.
A spokesperson has indicated that the Government was working from modelled scenarios in which vaccines remain moderately effective and in which active testing, tracing and isolation continues to be used in a manner similar to which it is used today.
The modelling didn't look at the use of lockdowns in particular, but said these could be called for if vaccination rates were low enough and the health system was still being threatened with collapse due to the case burden.
