Coronavirus trajectory revealed to Cabinet via expert analysis

Modelling presented to the Cabinet subcommittee on Covid-19 on Monday night shows the potential path of infection in the weeks ahead, which could put the healthcare system under near unprecedented levels of pressure.

It was published last night on Twitter by Prof Philip Nolan, chairman of the Irish Epidemiological Modelling Group, an advisory group to the National Public Health Emergency Team. But what does it mean?

There are several key slides that describe two scenarios. One optimistic, the other pessimistic. The first couple show how many cases we could have, on a daily basis.

There’s two key things to note.

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The first relates to how much acquired immunity there is in the population – ie, people who aren’t susceptible to infection because they’ve already had the disease. We’ve found about 500,000 cases, but obviously not every one is diagnosed. The more “undiagnosed” cases there have been, the more acquired immunity there is. In the optimistic scenario, it assumes that for every four cases detected, there’s another six undetected – so, more resilience through acquired infection.

In this scenario, the virus hits that wall of acquired immunity, plus immunity through vaccination, causing it to peak in early December at about 5,000-6,000 cases per day, descending through to late January thereafter.

The pessimistic scenario assumes there’s four infections undetected for every six detected. That level, combined with the vaccines, provides less overall immunity, which would mean a bigger wave.

Under that pessimistic scenario, daily case numbers peak at a little over 12,000 per day in the middle of December, and then again begin to decline.

The second thing to note is a bit simpler: these assume social contact is held constant through the next few weeks – not declining now because of restrictions or a change in behaviour (ie, seeing each other less), and not increasing in December, when people tend to see each other more.

The other slides show how this would affect the hospital system. Optimistically, this peaks at about 1,100 hospitalisations during December, and a peak intensive care (ICU) occupancy of 200, just below the level of ICU admission with the virus during January. We shouldn’t assume that this would be less damaging than the January wave. First of all, it’s a small margin, and second, that January wave meant a lot of people getting ICU-style treatment outside ICU in order to keep those beds free for non-Covid-19 care. Third, the HSE is trying to do as much non-coronavirus care as possible, whereas an awful lot of this was just called off in January. That makes things harder.

ICU situation

In a pessimistic scenario, the peak of hospitalisations is above the January wave, at some 2,250 (January topped out at 2,020), and significantly above the ICU occupancy. The models suggest that about 450 people would be sick enough with Covid-19 to need an ICU bed. Again, in reality, the healthcare system would need to flex to accommodate this, as this is more than the country’s intensive care unit capacity. You’d get a lot more people receiving care outside ICU if these numbers came to pass, which, obviously, is bad.

So, what’s going to happen? The truth is nobody knows for certain – models show a range of outcomes, they’re not predictions. And they’ve been off before, because the pandemic is influence by a whole range of things, principally human behaviour. So it’s literally in our hands (again). Ministers were told on Monday that the most likely outcome lands somewhere between the two, specifically about 1,000-2,000 people in hospital and 150-250 needing ICU, with more on advanced respiratory support outside hospital.

As everyone has been saying, all the above are grim scenarios.

What about waning immunity?

As Prof Nolan noted last night, the vaccines remain effective but there is a waning effect. They continue to protect against people getting sick, but less so against people just catching the disease. So, that means there’s a lot more Covid around, building up a bigger force of infection. Fewer people are being hospitalised, but there’s still enough people catching the virus for a small percentage of that number to be a big problem for hospitals.