In New Zealand, unusually, Covid-19 has so far improved population mortality. But what does that mean for the future?

We are all aware of the illness and deaths brought by the Covid-19 pandemic across the world.  The number of deaths attributable to the pandemic is difficult to count and compare across countries due to incomplete testing, definitional inconsistencies and the judgements required to attribute non-Covid deaths indirectly caused or avoided due to the pandemic or reactions to it (such as lockdowns).

The number of “excess deaths” has become a well-used indicator of Covid-related mortality.  Excess deaths are tracked weekly in many countries.  Excess deaths measure the actual deaths occurring compared to what would have been expected on pre-pandemic levels of mortality.

There are different ways of counting excess deaths. Two examples are:

• The Continuous Mortality Investigation (CMI) of the UK’s Institute and Faculty of Actuaries tracks the number of actual deaths registered in England & Wales in a week less the number that would have been expected if Standardised Mortality Rates (SMR) had been the same as in the corresponding week of 2019.  SMRs adjust for changes in the size, age and gender distribution of the population over time so that the excess deaths calculation focuses on changes in death rates. The CMI estimated around 66,200 more deaths for England & Wales from the start of the pandemic to 1 January 2021 than if mortality rates were similar to those experienced in 2019. 
• In a study of 29 high-income countries published in the BMJ, comparing observed deaths to the number expected from a model based on data from 2016-19, England & Wales was estimated to have 83,900 to 86,800 excess deaths in 2020.

Clearly, different methods of counting can lead to different results, but whichever method is used, there have been many lives lost prematurely.

Negative excess deaths in Aotearoa

Aotearoa New Zealand was one of only three countries in the BMJ study estimated to have negative excess deaths in 2020, that is, fewer deaths than expected. The mortality rate in New Zealand was significantly lower in 2020 than previous years, which resulted in around 2,500 fewer deaths than would have been expected by the estimate of this study. The other countries estimated to have negative excess deaths were Denmark (around 160 fewer deaths in a population 20% larger than New Zealand’s) and Norway (around 70 fewer deaths in a population 12% larger than New Zealand’s).

The actual number of deaths confirms the lower mortality experience in New Zealand in 2020.  There were 32,613 deaths, which is slightly fewer than the average of the preceding five years, a period when the population was growing. Aotearoa had only 25 deaths attributed to Covid-19 in 2020, and has had 27 such deaths to date (September 2021). A Covid-19 dashboard is here.

A significant factor in this reduced mortality in New Zealand was the near absence of influenza in 2020, as closed borders prevented flu getting into the country and lockdown stopped transmission.

Comparing the change in death rates in Aotearoa New Zealand with those in England & Wales since 2000-1 highlights the different experiences.  The chart below shows the change in an all-ages mortality rate across the population, each standardised for changes in population size, age and gender distribution.  Mortality rates, also called death rates, fluctuate each year as seasonal impacts and random variations occur. There are usually more years when age-standardised death rates reduce, rather than increase, consistent with gradually improving mortality.

The large values for 2019-20 for both countries indicate an extreme event.  For England & Wales the value is a large positive, meaning a worsening mortality rate. For Aotearoa, the large negative mortality rate shows a large mortality improvement in 2020

Oldest ages fared well

The change in death rates in Aotearoa New Zealand for each year since 2000-1 is shown by age group, for ages 50 and over, in the chart below. The orange line of 2019-20 is at the centre of the spider’s web, indicating the large negative change in all age groups as mortality improved over the year. 

While mortality improved significantly for all these age groups in 2019-20, the orange line is closest to the centre for the oldest age groups.  Older people were well cared for through the pandemic and achieved some of the largest annual mortality improvements seen over the last 20 years.

Implications for life expectancy

Period life expectancy provides a snapshot summary indicator of population health at a point in time.  It assumes that people will experience the population average death rates as they were in the period in question at each age for the rest of their life.

If the period is 2020, period life expectancy will be higher in New Zealand than in previous years, although national period life tables are calculated over a 3-year period to smooth statistical variations year on year. In countries with a high death toll from Covid-19 period life expectancy will be lower than in previous years. These calculations are already being made, for example:

Between 2018 and 2020, life expectancy in the US decreased by 1.87 years (to 76.87 years)

BMJ 2021;373:n1343

There is more uncertainty than there has been for a long time about the future trend of death rates. For example:

• The headline death rates directly attributable to Covid-19 in most countries are expected to decrease because of vaccinations, growing population immunity, and public health measures. 
• However, the pandemic may leave a long tail of higher-than-expected mortality, because of weakened physical or mental health from the virus itself or from the effects of lockdown or associated economic stress, including if that impacts badly on healthcare provision. 
• On the other hand, there may be some mortality improvements if the pandemic increases health awareness and healthcare providers are better funded.

The uncertainty of the effects of Covid-19 on future mortality makes predicting future trends in life expectancy difficult and complicated by varying impacts at different ages and over time. It is possible that reported period life expectancy temporarily reduces, or blips up and down, while cohort life expectancy continues increasing steadily. For example, say the pandemic affects death rates at each age for three calendar years:

• Calculations of period life expectancy at birth for those three calendar years will use each of those affected death rates, for each age. 
• The calculation of cohort life expectancy at birth will only use three of those affected death rates, for the ages the cohort lived through in those three calendar years.  For the other 100 years or so of the cohort’s lifespan, the death rates used will be the actuals, for the ‘lived’ ages, or estimated for later ages in future years. The pandemic effects will have less weight.

Interpreting headlines on “life expectancy” will require care. The pandemic could well provide a teachable moment to increase understanding of the distinction between period life expectancy as a useful snapshot of population health and cohort life expectancy as an indicator of likely lifespans for a birth cohort of people.