Colleagues from GSTT in collaboration with the Royal Free and St. George’s have just published a new paper evaluating the transmission dynamics of Omicron SARS-CoV-2 vs. previous variants in hospitalised inpatients in Infection Prevention in Practice. The key finding is that the median days to inpatient contact conversion were significantly fewer for Omicron SARS-CoV-2 when compared with previous variants (median 3 days for Omicron vs. 4 days for previous variants).

In this retrospective cohort study, any patient identified as a “COVID-19 contact” in electronic records eligible for inclusion. A total of 5,156 COVID-19 contacts were included, and 476 (9.2%) of these converted to SARS-CoV-2 PCR positive during the 14 days following their exposure. The study included patients from January 2020 to December 2022. The median days to conversion were 3 days for Omicron, 3.5 days for Delta, 4 days for Alpha, and whopping 7 days for the original “Wuhan” variant. For the purposes of statistical comparison, non-Omicron variants were grouped together, and the median days to conversion were significantly fewer for Omicron (3 days) vs. non-Omicron variants. It’s quite remarkable to see the median days to conversion almost cut in half when comparing the Omicron and Wuhan variants!

Some points for discussion:

• Why were the median days to conversion fewer for Omicron vs. previous variants? We know by several measures in several studies that SARS-CoV-2 variants have become progressively more transmissible and less virulent. I don’t think the transmission routes have changed – or at least they have not changed significantly. But rather the interaction between the virus and host cells has changed in favour of earlier infection (see this excellent review for more details on this).
• The overall contact conversion rate was lower than you’d expect to see. Other studies reported higher contact conversion rates, although this will depend on the setting. For example, it can be >20% in household settings compared with 9% in our study. However, it’s important to note that we did not undertake active followup of the contacts. Some were not tested at all during the 14 days after their exposure, as policies and practices changed over the years of the pandemic. And some of the contacts did not stay in hospital for the full 14 days. If we had done active followup of contacts, I’m sure the contact conversion rate would have been considerably higher.
• Why did we see an increase in contact conversion in the tail end of the 14 day period? The answer here is most probably secondary exposures. When contacts are cohorted together in a bay (a very frequent occurrence), they are re-exposed when somebody else in the bay coverts to SARS-CoV-2 positive.
• Variants were assigned either by typing or by inference based on the predominant circulating variant – so there could have been a bit of variant misclassification going on.
• The study has important applied infection prevention and control implications. It has been challenging throughout the pandemic to find ways to isolate contacts of cases in order to limit in-hospital transmission of SARS-CoV-2. In the early days of the pandemic, and as supported by the findings of this study, we frequently saw ‘late’ conversion of contacts (remember, the median days to contact conversion for Wuhan was 7 days). This meant that hospitals suffered operational challenges, to say the least, in accommodating large numbers of COVID-19 contacts, especially during prevalence peaks. The good news from this study is that if a contact is going to convert, they are likely to do it in the first few days following exposure with Omicron, which means we can step down isolation of contacts much sooner than we could before.  

It’s great to see this piece of applied research published, and I hope it will be useful to inform our day-to-day decision making and strategy going into the colder winter months.