With the first cases reported in March of 2020 in both countries, mitigation efforts kept cases rates low (relative as compared to the massive Omicron outbreak) until a clear January 2021 peak.  While there was not much genetic epidemiology being done at that time, the alpha variant was seen in March 2021 in 97% of UK cases and 15% of US cases. Although the UK had a more sharp peak than the US, it is clear that case rates in that instance accelerated before the UK.

The next clear rise in cases occurred in the summer of 2021 – fueled by the Delta variant.  In this instance the UK did take the lead with cases peaking the week of July 21st while the US topped out September 5th. Delta also began circulating first in the UK with 53.5% of cases in May being due to the variant as compared to 3.9% in the US.  By August 31st, 99.95% of cases in the UK and 99.44% of cases in the US were due to Delta.

The next big surge was Omicron, with the UK again taking the lead. Cases began to rise in the UK in late October and peaked the week of January 5th. The US had cases rising late November with a peak the week of January 14th. But this isn’t quite as simple (but still follows a pattern of the UK taking the lead) as one might think when considering the genetic epidemiology.  

In the UK in October, 99.97% of cases were Delta. In mid-December, over 60% of the cases were still Delta (40% Omicron, BA.1) and in early January 10% were Delta, 90% Omicron.  

In the US in October, 99.78% of cases were Delta. In mid-December, over 75% of the cases were still Delta (25% Omicron, BA.1) and in early January 16% were Delta, 84% Omicron.  

So what really occurred was a surge fueled by a mix of Delta and Omicron (BA.1) with the latter supplanting Delta as the primary circulating variant. Omicron took hold in the UK before the US, however.

The UK experienced a second Omicron wave in March of this year fueled by the BA.2 variant – something which simply did not occur in the US. In mid-March, 75% of cases in the US were still BA.1, 25% BA.2 as opposed to the UK where the opposite was seen: 20% of cases were BA.1 and 80% BA.2. Cases rose in the US in May, this time due to the BA.4 variant (accounting for 56% of cases and only 4% were BA.4) – a pattern which did not occur in the UK (80% of cases in the UK remained BA.2).

Dr. Tim Spector, professor of genetic epidemiology at Kings College London says, “Generally, what happens in the UK is reflected about a month later in the US.” Is this true?  Well sort of. It was not true for the Alpha variant, was most certainly true for Delta and again for Omicron. Interestingly, the distribution of BA.1, BA.2 and BA.4 has varied significantly during 2022 leading to a more short term trend where the US and UK are quite out of phase (Figure below).