Unlike other outbreaks, where we cannot predict future super-spreading events, there are two potential super-spreading events that are known, Christmas and New Year. While the incubation period of SARS-COV-2 is 14 days, 90 per cent of people develop an infection within 5-6 days, and are the most highly infectious right at the start of the infection.

The expected surge in cases on December 30, five days after Christmas, likely reflects infections acquired on or around Christmas day. More infections acquired on December 25 will keep occurring up to January 3. So, the Sydney Test falls right after super-spreading event #1, Christmas Day, and superspreading event #2, New Year’s Eve – by January 7 we would expect to see a rise in cases related to NYE. To add to the risk, there are now clusters outside of the northern beaches, including in Croydon and Wollongong, as well as mystery cases whose source of infection is unknown and positive wastewater in Hornsby, the Central Coast and Liverpool.

Being outdoors is a mitigating factor for the cricket. Indoor gatherings are far more risky than outdoor gatherings, so sitting in a grandstand watching the match is safer than gathering with 50 people in a pub to watch it on TV. This is because respiratory aerosols, which carry the virus, accumulate in poorly ventilated indoor settings and amplify risk as the time spent in such a venue increases.

However, the risk of close person-to-person spread is still present outdoors, and can only be mitigated by social distancing and face masks. Other than close contact, the greater risk for the third Test is the crowds of people getting to and from the SCG on buses and trains. Without any mask mandate, this is especially high risk, as numerous outbreaks of COVID-19 related to mass transport vehicles have been documented.

In addition, people crowding together to purchase food and drinks during the match, as well as using the toilets, are at risk. SARS-COV-2 is shed in faeces, and research shows that toilets can be highly contaminated with the virus. The risk is not just from contact with surfaces, but also from aerosolised faecal material after flushing, so wearing a mask in the toilet can reduce that risk. The shouting and singing that are common at the cricket are also a risk – simply breathing and speaking generate respiratory aerosols, but this is amplified with shouting or singing as we have shown.

Finally, we have been advised there is no rush to make vaccines available in Australia until March, and there is no goal of mass vaccination. Instead we are told we should expect to live with COVID-19 long term. I do not agree with this assessment, as with a vaccine of high enough efficacy, we could achieve elimination of COVID-19 in Australia.

COVID-19 vaccines may even be useful as post-exposure prophylaxis during outbreaks, and we could be exploring this indication right now. At the very least we should be vaccinating health workers and aged care workers as soon as possible so that they are protected during outbreaks. However, currently, the majority of the population is unprotected and non-immune to the virus, meaning outbreaks will continue to occur.

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My prediction is the greatest danger time for Sydney will be from January 6-14. This is because New Year’s Eve could be a bigger risk than Christmas. The majority of people who got infected on Christmas Day will be at their peak of infectiousness on New Year’s Eve and New Year’s Day. A Test match starting on January 7 in the midst of a surge in cases, would be a risk. The match can still be played but without live spectators, just like the fireworks and Carols in the Domain.

On our journey across this dangerous tightrope, without the full armoury of protections we could be using, such as mask mandates and vaccines, why add yet another potential super-spreading event to the mix so soon after Christmas and New Year?

Professor Raina MacIntyre is head of the Biosecurity Research Program at UNSW.

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Professor Raina MacIntyre is the head of the Biosecurity Research Program at the Kirby Institute, UNSW Medicine.

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