Thread by @jeffgilchrist

COVID-19: Ontario and UK early estimates of Omicron severity

Both Ontario and the UK recently released reports on early estimates of Omicron severity with the UK also looking at data including children. H/T: @Billius27 & @ASPphysician 🧵1/

First, Ontario used a matched cohort study with 6,314 Omicron cases matched with at least 1 Delta case based on age, gender, and onset date ( publichealthontario.ca/-/media/docume… ). 2/

A big caveat to their study is the median age of cases was 30 for Delta and 28 for Omicron so a study population that was on the lower end of hospitalization risk in the first place. 3/

Taking into account vaccination status, the risk overall of hospitalization or death was 54% lower in Omicron cases. Between Nov 22-Dec 17, 2021 they found Omicron had a 0.33% hospitalization and 0% death rate compared to Delta with a 2.24% hospitalization and 0.3% death rate. 4/

Looking at people younger than 60, the risk of hospitalization or death was 70% lower with Omicron. With few people in their study over age 60 they have a preliminary estimate of 45% lower risk for people over age 60. 5/

Delta which the study was comparing to already had a much higher risk for hospitalization, ICU admission, and death, so the 54% lower risk for Omicron still puts it as more severe than the 2020 version of COVID-19 ( cmaj.ca/content/193/42… ). 6/

The Ontario study highlights, "Due to the transmissibility of Omicron, the absolute number of hospitalizations and impact on the healthcare system is likely to be significant, despite possible reduced severity." 7/

What does that mean? Despite Omicron having 54% lower risk for hospitalization and death, the massive increase in people infected more than cancels out this reduction in risk. In reality many more people can be hospitalized with Omicron than Delta. 8/

If Omicron is 6x more transmissible like some estimates have it, then a 54% reduction in hospitalization still means 2.8x more people could be hospitalized with Omicron compared to Delta (6 * 0.46 = 2.8). 9/

This is of course much better than 6x more people being hospitalized, but a variant that can hospitalize more people than Delta and the original variant I wouldn't describe as "mild". 10/

The UK released a technical briefing on Omicron hospitalization for cases up to Dec. 29, 2021 which include almost 200,000 confirmed Omicron cases so much more data than the Ontario study ( assets.publishing.service.gov.uk/government/upl… ). 11/

There were 815 individuals admitted to hospital with an age range from 0 to 100 years, median 45.5 years with 61% > 40 years and 31% > 70 years of age. A total of 57 people died from Omicron within 28 days with the median time of death from testing being 5 days. 12/

The study found the risk of presentation to emergency care or hospital admission was 47% lower for Omicron than Delta, and the risk of hospital admission alone was 67% lower than Delta taking into account age, sex, ethnicity, travel, vaccine status, and local socioeconomics. 13/

How well do the vaccines protect against Omicron and Delta from hospital admission (see Table 4)? 14/

Compared to someone who is unvaccinated, the vaccines reduce hospital admission risk within 14 days of positive test by:

1 dose = 0% Omicron and 58% Delta
2 doses = 65% Omicron and 82% Delta
3 doses = 81% Omicron and 85% Delta

15/

While 1 dose seemed to provide decent protection from severe disease with Delta, they didn’t really find any difference between those who were unvaccinated and those with 1 dose of vaccine. 16/

The increased immune evasion from Omicron means that 3 doses provides much more significant protection from hospitalization with Omicron than 2 doses compared to the difference between 2 and 3 doses with Delta. 17/

This analysis was not adjusted for co-morbidities and does not distinguish between in hospital severity which takes longer to assess. 18/

The UK report also notes, "It is important to highlight that these lower risks do not necessarily imply reduced hospital burden over the current epidemic wave, given the higher growth rate and immune evasion observed with Omicron." 19/

The UK has some very preliminary data that the risk of hospitalization for school-aged kids (5-17) is about 58% lower than Delta. That means the previous risk of about 1% of child cases being hospitalized for COVID-19 infection would drop to around 0.42%. 20/

Recall that Omicron infects many more people, far more than this reduction compensates. For every million children that get infected with Delta you had around 10,000 children hospitalized whereas with Omicron it would be something like 4,200 children. 21/

But if Omicron is 6x more transmissible and 6 million children were infected instead of 1 million you end up with 25,200 children hospitalized, a much higher number of children than Delta would cause even with its more severe infection. 22/

The net impact would be more than 2.5x as many children could be hospitalized with Omicron due to its much higher (6x) transmission rate. 23/

This doesn't even take into account that the COVID-19 virus gets into many parts of the body and can cause damage, potentially long-term, and potentially permanently. 24/

Unfortunately we don't really know anything yet about Long COVID and Omicron, whether it will be similar to other variants, better, or worse. 25/

Information about Long COVID and previous variants in children found, "Persistent symptoms of loss of smell, headaches, cognitive difficulties and sore throat and eyes each occur in 2%-8% more children and young people (CYP) after SARS-CoV-2 infection..." 26/

"...than in those without infection. Two large controlled studies suggest that 5%-14% may have multiple persistent symptoms 4 weeks or more after acute infection." ( journalofinfection.com/article/S0163-… ) H/T: @ASPphysician 27/

The study found that Long COVID symptoms were more likely as age increased so hopefully vaccination helps protect against Long COVID like it does severe disease since the vaccination strategy focused on older children first and is moving downward in age. 28/

For example, persistent symptoms in children were 2.74x more likely in children aged 6-11 and 2.68x more likely in those aged 12-18 than children less than 2 years of age. 29/

When looking at symptoms longer than 28 days, 5.1% of children 12-17 years developed these compared to 3.1% of children 5-11 years. Persistent symptom prevalence was 1.79x higher in females, 2.67x higher in teenagers, and 2.95x higher in CYP with long-term health conditions. 30/