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Covid 19 Omicron outbreak: New modelling – reinfections could be 46 per cent of cases in second wave

New modelling predicts that a second Omicron wave in the second half of the year will likely be lower than the one...

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New modelling predicts that a second Omicron wave in the second half of the year will likely be lower than the one that peaked in March and put hospitals under pressure. Photo / Sylvie Whinray

New modelling suggests that a second wave of Omicron could hit in the second half of the year, driven in part by people getting reinfected because their immunity has waned.

The wave is estimated to peak at fewer daily cases than the first wave, as well as fewer hospitalisations – unless infections were prevalent among higher-risk people such as the elderly.

In this scenario, up to 46 per cent of cases had already had Omicron earlier in the year, according to the paper published today by Covid-19 Modelling Aotearoa.

The paper also found that higher transmission – either due to further relaxing of public health measures, or more risky behaviour – didn’t necessarily lead to higher peaks.

“However, they generally resulted in more cumulative cases and sustained demand on healthcare systems (more than 250 hospital beds occupied throughout the winter period),” said the paper, which is yet to be formally reviewed.

The first Omicron wave peaked at different times in different parts of the country, but nationally the peak in daily cases was just over 20,000 on March 5, with daily hospitalisations peaking at just over 1000 in late March.

The health system was described as stretched by not overwhelmed, though it came under varying pressure at different times; even today, well beyond the peak, planned care including some major surgery has been deferred in Christchurch as the hospital grapples with many staff away sick.

Earlier modelling about the second wave had assumed that people who caught Omicron would not get reinfected.

“In reality, infection-derived immunity against SARS-CoV-2 likely wanes over time,
meaning it is possible for people to be infected more than once,” said the paper.

“Mathematically, this means that transmission of the virus does not die out and instead gives rise to a stable equilibrium state in which a steady fraction of the population is infected with the virus per day.”

The paper looked at four scenarios of varying levels of people contact – including one with increased contact among older people.

It then looked at waning immunity at a baseline level (still at around 75 per cent effective against reinfection 20 weeks after testing positive), as well as a slow-waning (85 per cent effective) and fast-waning level (60 per cent effective).

Peak daily cases were lower than in the March peak in all scenarios, as were peak daily hospitalisations except for the one with increased people contact among older people.

“In these scenarios, peak hospitalisations and total deaths exceeded the first wave. Although the overall distribution was still dominated by under 35s, relatively small increases in the proportion of cases in over 60s had a major impact on hospitalisations and deaths due to the steep age gradient in risk.”

2022 totals for infections, cases, hospitalisations and deaths under different scenarios of people contact and waning immunity. Source / Covid-19 Modelling Aotearoa
2022 totals for infections, cases, hospitalisations and deaths under different scenarios of people contact and waning immunity. Source / Covid-19 Modelling Aotearoa

The total Omicron death toll for 2022 in this scenario is estimated to be between 2500 and 4060, depending on the speed of waning immunity. Between 37 and 46 per cent of the total cases are estimated to be reinfections.

The paper notes that similarly higher numbers of hospitalisations and deaths could happen with increased contact among other higher risk groups, including Māori, Pasifika or the immunocompromised.

This scenario is less likely to transpire, however, as a second booster dose will be available for vulnerable people, which will boost their immunity. The Government is expected to announce the details of exactly who will be eligible next week.

The death toll range for 2022 for lower levels of people contact was between 1210 and 1400, where the proportion of cases that are reinfections ranged from 20 to 28 per cent.

The paper notes that a second wave could be worse than these estimates if the assumptions underlying the model underestimate how much immunity will wane, and therefore underestimate how many people will get reinfected.

A lack of worldwide population-level data means that the actual level of waning immunity is still unclear.

The potential spanner in the works is an Omicron subvariant that can more easily escape immunity from previous Omicron infection.

There is already emerging evidence that if you’ve already had BA.1 or BA.2, which were dominant in the first wave, you’ll have good levels of immunity against those subvariants for at least three months, but you’ll be more easily reinfected by subvariants BA.4, BA.5 or BA.2.12.1.

These three subvariants have been detected in New Zealand, and are already behind surges in case numbers overseas, including in Europe and the US. While they appear to have a growth advantage over BA.1 and BA.2, so far do not appear to lead to an increased risk of severe illness.

The model does not account for a new subvariant or seasonality, and it notes that the impact of a second wave will disproportionally impact communities with lower vaccination rates and worse access to healthcare.

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