Covid-19 interventions laid bare by Omicron

The most transmissible variant of SARS-CoV-2 is exposing secrets about the effectiveness of different non-pharmaceutical interventions.

All measures targeted at reducing transmission can be classified into 3 groups:

1. Targeted reduction of contacts of those high risk of being infected: isolation of infected people and quarantining of their contacts

2. General reduction of contacts, agnostic of infection risk: blanket restrictions on meeting others and closing sectors of the economy (e.g. night clubs)

3. Making contacts between people lower risk: Vaccines, masks, social distancing

Some countries made it through most of the first 2 years of the Covid-19 pandemic without needing to regularly utilise the second group, often referred to as “lockdown” style measures. These countries include the likes of South Korea, Taiwan and Hong Kong. Their successes are usually put down to the high uptake of masks, focussing on ventilation, and highly functioning test, trace and isolate procedures (TTI).

With Omicron however, things have been very different. These countries have seen record numbers of cases, sometimes exceeding peak relative case numbers seen previously in hard hit western countries. In the case of Hong Kong, this has also resulted in many deaths due to low rates of vaccination in their elderly population.

What can this tell us about preventing high risk contacts (e.g. TTI), or measures to make contacts safer (e.g. masks)? Omicron is transmitted in the same way as other variants of SARS-CoV-2, so we wouldn’t expect masks to be significantly less effective. Yet the virus has run rampant despite high levels of compliance, including high quality masks.

What has changed is the generation interval. This is the time between one person becoming infected and then subsequently infecting someone else. For previous variants virus, this was around 4-5 days. Omicron has shaved 2 days off this time, and the generation interval is now thought to be around 2-3 days. This means Omicron gets a 48h head start on contact tracers every time a new infection occurs. As we know from exponential growth, transmitting twice as fast is more effective than transmitting to twice as many people per infection.

Omicron has beaten the contact tracers, and this is likely why it’s growth has been so explosive in previously unaffected countries. It looks as though their previous success was due mainly to the effectiveness of their TTI procedures and little to do with mask compliance or other measures. Once Omicron could outpace it, it was free to spread, and has caused huge amounts of infections as a result.

This is an important lesson for the future. It shows us that TTI is extremely effective when it is done properly, at relatively low levels of infection, and the generation interval is long enough (more than 3 or 4 days). Faster than that (especially with pre- or asymptomatic transmission) and different strategies are needed.

When it comes to TTI vs other measures such as face coverings for Covid-19, there is no competition.