The current outbreak in Sydney may take months to recede, according to Guardian Australia analysis of Covid-19 waves across the OECD.
It took more than 80 days on average to go from the peak to the end of the wave in the 80 outbreaks examined. The average is longer this year – over 90 days – than the 75 days in waves that peaked last year.
And the Sydney outbreak may not have peaked yet, with the New South Wales premier warning that case numbers will likely keep rising after daily recorded infections eclipsed 100 several times over the past week.
Experts say there are many reasons for the differences in severity and length of Covid waves across and within countries. This includes differences in population size and density, as well as government responses such as vaccinations, closing schools and retail, and issuing stay-at-home orders.
Other factors include where outbreaks began, as well as the different variants that have emerged throughout the pandemic. The magnitude and length of the waves themselves also affect each other, as rising case numbers can increase the number and types of locations where transmission occurs.
Prof Allan Saul from the Burnet Institute estimates that Sydney’s current outbreak could extend “well into September” if it followed the same trajectory as Melbourne’s did last year. But there isn’t yet enough data to make a prediction, and there are many factors distinguishing the Sydney outbreak from Melbourne’s second wave, such as new, more transmissible variants.
“We’re dealing with a Delta outbreak. We’re dealing with different controls than what we saw in Melbourne. I don’t know what is going to happen in the next week,” Saul says.
Guardian Australia’s data analysis looked for the highest and lowest points in a 60-day moving window for each country. We identified 80 distinct waves across the OECD, with an average total length of 167 days.
Australia has had two of the smallest outbreaks in the data. New Zealand has the smallest and 10th smallest outbreaks in the dataset.
Slovakia, Sweden and the United States had outbreaks that took the longest to go from peak to trough – over 160 days each. The United States’ longest outbreak, which saw over 300,000 new cases per day at its peak in January, ended only very recently.
Australia was an outlier in that its waves began and ended at zero cases. Most countries have not come close to eliminating community spread and so experienced a series of waves that rolled into each other.
The latest distinct wave in the US started at a minimum of over 40,000 new cases per day and ended in June at over 10,000 new cases per day.
‘A feature of Delta’
Prof Catherine Bennett from Deakin University says the Delta variant may have changed the kinds of locations where transmission occurs. Workplaces were key in Melbourne’s second wave last year, but Delta’s higher viral load may make it spread more efficiently.
“Where before it took about 15 minutes, now we see that you just have to be standing next to someone in queue, indoors, wearing masks, and you might still get infected,”Bennett says.
“So that’s the problem – that this new variant might introduce more of those casual exposures. It doesn’t necessarily mean [transmission] outdoors but we’ve seen that in the right settings.
“It certainly increases the range of people around a case who can become positive. And that then increases the range of sites that can become transmission sites. So that is something that is shifting over time.”
More infectious variants may also lead to steep waves, as the virus spreads through the community.
No two outbreaks are the same
The differences in where outbreaks occur, and the socioeconomic dynamics of those places, can also have a huge impact on magnitude and length. Australia has so far managed to eliminate local transmission several times and so outbreaks are being seeded through quarantine each time.
But in many other countries, outbreaks are first established in densely populated areas where people can’t socially distance.
“The nature of the outbreak is also defined by how people live and move in a city, density, but also where the virus lands,” Bennett says.
“Those places where it was the workers in Asia and they had these big outbreaks associated with dorms – that’s where you get not only rapid spread but also an outbreak that is hard to control because people can’t isolate.
“We saw a different story in eastern Sydney than south-western. And that was partially because vaccination rates in the elderly are lower in south-west Sydney. It’s partially because you’ve got larger households.”
Saul notes that Melbourne’s extended wave last year took 91 days to go from the peak to zero cases and about 58 days to go from the peak to 10 cases.
This was similar to the 92 days estimated by Guardian Australia’s analysis, with the two significant Sydney outbreaks taking 54 days and 32 days to wind down, respectively.
Researchers at Oxford University have found that many countries reacted quickly and early in the pandemic, instituting stay-at-home orders and other restrictions. But many governments have become less responsive to cases and deaths.
Data from the Oxford University policy tracker shows that policy responses vary wildly across countries and have only diverged more over time.
“In Victoria in the second wave, if you look at the data all the way through, [infections] were linked either to the workplace or residential care. And community outbreaks were just on the fringe because we kept reseeding virus back,” Bennett says.
“If we had shut down aged care or shut down the virus in aged care completely within the first month, our second wave would have been six to eight weeks long.”
Notes and methods:
Covid case data sourced from Our World in Data for national level and CovidLive.com.au for subnational Australian data.
Locally sourced infections for Australia identified by subtracting overseas infections from total daily infections, with negative values removed. Data was smoothed using a seven-day rolling average.
Waves were identified in the smoothed data by finding maxima and minima in rolling 60-day (for national data) and 30-day (subnational) windows for each jurisdiction. Waves were then defined as the days between two minima points with a maxima in between.
Waves with minima before April 2020 were not included in Australian subnational analysis as data with distinct locally and overseas infections is not available.
Ongoing waves were not included in the analysis.