Is the Omicron subvariant BA.2 more transmissible?

Since its emergence in late 2021, Omicron (or BA.1) has quickly become the dominant variant of the COVID-19 virus. The mutations it harboured meant it was more transmissible than the Delta variant, so was able to spread rapidly through populations around the world. It has been found to confer a milder illness than Delta – though milder should not be mistaken for mild.

Scientists are now tracking a subgroup of the Omicron variant, known as BA.2, which was first identified in India and South Africa in late December 2021. Since then, it has been found in the US and Europe. The subvariant is thought to have 20 additional mutations on its spike protein compared with BA.1, though scientists are not quite sure what this will mean for the course of the pandemic. So far, there is insufficient evidence to determine whether BA.2 causes more severe illness than BA.1.

The reason why some people refer to BA.2 as the “stealth” variant is because they claim it is not as easy to detect on PCR and lateral flow tests. This is in fact not true; it can be detected on both types of tests.

PCR tests look for three different DNA sequences when detecting the SARS-CoV-2 virus and will show a positive result if at least two sequences are present. One of the sequences the test looks for is the S-gene, which is found on the spike protein. This is absent in the BA.1 Omicron variant and is known as the “S-gene dropout”; PCR tests will still return a positive result as they will detect the other two DNA sequences they are designed to pick up. But the absence of the S-gene means it was easy for scientists to differentiate between BA.1 Omicron and Delta, which had the S-gene present. Like Delta, the BA.2 Omicron variant has the S-gene present, so it is harder for scientists to tell the difference between BA.2 and Delta, hence the term “stealth” variant. But in another way, this might actually work in our favour: with Omicron now dominant and Delta quickly disappearing, any PCR tests that detect the presence of the S-gene going forward are likely to be the BA.2 subvariant of Omicron.

This variant has become dominant in Denmark. The speed at which cases have risen there and in other countries like the UK suggests that BA.2 may be more transmissible and able to outcompete the BA.1 variant. A recent study looked at the transmission of Omicron subvariants in Danish households and found that the BA.2 subvariant is substantially more transmissible than the original variant.

The study looked at households in Denmark who had one person test positive for Omicron between December 20, 2021 and January 11, 2022. They had 2,122 people who were positive with BA.2 and 6,419 people with BA.1. The researchers then followed these people to see if they passed on their subvariants of Omicron to other members of their household. They found that the BA.2 subvariant transmitted more in both unvaccinated and vaccinated members of the household when compared with the BA.1 subvariant. But they also found that people who were unvaccinated were more likely to transmit BA.2 than those who were vaccinated.

Eric Topol, an American cardiologist and scientist, and founder of the Scripps Research Translational Institute, said about the study that the BA.2 variant findings suggest triple vaccination protects well against symptomatic infection, but he also warned that the increased transmissibility will likely prolong the Omicron wave in some places.

The BA.2 lineage does not appear to cause more severe disease, so there is no reason to panic. It is important to note that it is not unusual to have subgroups of the same variant – that is the nature of a virus that is able to mutate and which has as many human hosts as Omicron does. Vaccines still offer protection against serious illness and, according to the study, also protection against transmission, so it remains important to take them up when offered.