A catastrophe in which millions of desperately sick patients will go untreated by an utterly overwhelmed hospital system seems to be unfolding before our eyes.

It's developing first in Beijing and possibly in smaller cities and towns that we're not hearing from. Unless the Chinese Communist Party reimposes severe controls, the rest of the country looks like following Beijing into disaster.

Politics surely played a part in the CCP's decision to give up its COVID-zero policy two weeks ago, a move that has at least worsened the eruption of infection. But perhaps the greatest contribution of politics was setting up an explosive situation.

No helpful data is being reported from Beijing. But I have some, based simply on having so many friends there - in statistical terms, a quite useful number of them.

From my friends' experiences, I can see that maybe one-sixth of Beijing's 22 million people were already sick with COVID-19 last week, just days after controls were lifted, and a further 50 per cent, more or less, had fallen ill by Thursday this week.

Two friends separately offered an identical bit of data on Thursday: 70 per cent of their workmates were sick.

The virus is racing through the population with almost unimaginable speed.

When Australia, led by NSW, dropped its last major pandemic controls a year ago, we initiated our exit wave: a mild flow of infection moved across the population, topping up vaccine-based immunity and delivering us into a living-with-COVID future.

Our exit wave built up only slowly, though it still stressed the health system; it peaked after about four weeks. Also, it involved only a fraction of the population; most people didn't come down with COVID until later in 2022.

Beijing's wave, by contrast, appears to have been incredibly steep, peaking only a week or so after the lifting of controls. And most people are immediately caught up in it.

We used to talk about flattening the curve, so the flow of sick people into hospitals did not overwhelm the capacity of our medical workers to deal with them. What will happen in Beijing's hospitals when half of the population is infected at the same time?

Plenty of journalists have made fools of themselves with amateur epidemiology over the past three years, especially those who have compulsively looked for reasons for hysteria. Nonetheless, I must say I'm on the edge of my seat awaiting the next few days' news of hospital conditions in Beijing.

Then will come the rest of China, which had a lower level of COVID incidence when the controls came off but is surely building up steam very quickly.

How is this happening in a country that controlled the virus so well and kept up the fight for more than a year after other countries had moved towards living with COVID?

First, the virus is far more contagious than it used to be and is now really good at getting around immunity, says Adrian Esterman, professor of biostatistics and epidemiology at the University of South Australia. The average person infected with the original version found in Wuhan would infect 3.5 other people, he says. But for the latest strains, that number, called R, could be 10 or higher, maybe much higher.

Also, China has next to no natural immunity - as indeed Australia had almost none a year ago. But we had those superb Western vaccines, which are not bad at preventing mild disease and great at keeping people out of hospital.

Vaccination levels in China are not particularly high, especially among the elderly, and there has been a reluctance to get booster jabs. An exasperating myth in China insists that vaccination is unsuitable for old people.

Looking at the explosive growth of infection in Beijing, it's hard to imagine a late rush to vaccination can now do much good - unless the government slams on the brakes and locks everyone down again.

And Esterman notes that China is now in winter, so everyone is indoors, helping the virus to spread.

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The seasonal timing is proof that the CCP had not intended to initiate an exit wave now. Six weeks ago it still showed every sign of persisting with COVID-zero until at least the northern-hemisphere spring.

Probably two factors forced it to give up. One was that increasingly severe lockdowns and quarantine had frustrated the whole country; the party, for its own political wellbeing, could not risk protests getting out of control.

But its political motivations had also set up the problem. It had insisted on using only domestic vaccines, to support its relentless propaganda message that, under its faultless leadership, China does everything really well. Reports vary, but the consensus is that the Chinese vaccines are not as good as the Western ones that saved the rest of the world.

Maybe the party was waiting for better domestic vaccines.

It has certainly enjoyed filling local media with reports and pictures of foreign countries, especially the US, struggling with infections while China was serene.

But while it waited, its challenge was, predictably, getting worse: the virus was evolving towards greater transmissibility.

Last month, infections were already rising fast. Testing and tracing was getting harder, perhaps beginning to look impossible. Forces that the party had contained inside a great pressure cooker were building up.

And now: bang.