Rapid transmission makes reproductive number tracking obsolete: COVID-19 modelling

VICTORIA, B.C. – Due to the speed at which the Omicron variant is travelling through communities, Dr. Bonnie H…

VICTORIA, B.C. – Due to the speed at which the Omicron variant is travelling through communities, Dr. Bonnie Henry says data like the reproductive number and geographic representation don’t paint a clear picture anymore.

As a result, health officials are looking at other sources to evaluate the trajectory of the virus.

“As we’ve gone through this Omicron wave, we have transmissions that are happening very rapidly and we no longer are able to do that case and contact tracing because of the shortened incubation period that we’re seeing with this wave,” said Dr. Henry at Friday’s COVID-19 Modelling briefing.

“We also need to look at other sources to help us validate what we’re seeing and to understand the trajectory of what we’re seeing in BC compared to other jurisdictions.”

One source that health officials are using is the case rates over time and per cent positivity.

“Our test positivity has been very high and it’s been in the twenties for the last little while. We’ve seen in this past week particularly that we’ve started to see a decrease in the test positivity and the numbers of cases.”

Provinces like Ontario and Quebec are slightly ahead of British Columbia, according to Dr. Henry.

“We think that we hit the peak of community transmission in British Columbia probably this past weekend. That’s important. New hospitalizations are still a concern, and we need to understand who is it that’s being hospitalized, and what we can do to try and affect that trajectory in the next few weeks.”

While new cases are still on the rise in Northern Health, Dr. Henry says the highly populated areas of B.C. are driving the epidemic curve.

In terms of vaccine effectiveness against Omicron, Henry says it’s important to get booster shots.

“Even with two doses, there’s still a risk in older people because we don’t mount a strong immune response. Even if you’re vaccinated with two doses, it’s really important to get your booster dose.

And one of those reasons why we have really focused on our booster dose program on older people and people who are immune compromised and are more at risk is because we know that extra dose makes a big difference.”

Dr. Henry showed a graphic that explains why the proportion of vaccinated people in hospital is higher than the unvaccinated population.

“I know when we look at the numbers every day and there’s been a lot of people who’ve been remarking on the fact that there’s more vaccinated people in hospital than unvaccinated people, but that number comes from a much, much bigger pool of people.”

Dr. Henry pointed to one positive sign with Omicron’s characteristics.

“Most people aren’t getting a severe enough illness that they need to be hospitalized. That’s the good news. The challenging part is understanding where we are on the trajectory in terms of transmission in our community, and also what is going to be the impact on our healthcare system.”

While this variant hasn’t resulted in as many hospitalizations as Delta for example, Henry says it is causing healthcare staff to become sick and miss work, leading to shortages.

“Healthcare workers having to be at home, recovering from their illness has led to, staff being off ill in higher numbers than ever before in this pandemic.”

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