Scientists
are now tracking a rise in cases caused by a close cousin known as ba.2, which
is starting to outcompete ba.1 in parts of Europe and Asia. The following is
what we know so far about the new subvariant:
“STEALTH”
SUBVARIANT
Globally,
ba.1 accounted for 98.8 percent of sequenced cases submitted to the public
virus tracking database GISAID as of Jan 25. But several countries are
reporting recent increases in the subvariant known as ba.2, according to the
World Health Organisation.
In addition
to ba.1 and ba.2, the WHO lists two other subvariants under the omicron
umbrella: ba.1.1.529 and ba.3. All are closely related genetically, but each
features mutations that could alter how they behave.
Trevor
Bedford, a computational virologist at Fred Hutchinson Cancer Centre who has
been tracking the evolution of SARS-CoV-2, wrote on Twitter on Friday that ba.2
represents roughly 82 percent of cases in Denmark, 9 percent in the UK and 8
percent in the United States, based on his analysis of sequencing data from the
GISAID database and case counts from the Our World in Data project at the
University of Oxford.
The ba.1
version of omicron has been somewhat easier to track than prior variants. That
is because ba.1 is missing one of three target genes used in a common PCR test.
Cases showing this pattern were assumed by default to be caused by ba.1.
ba.2,
sometimes known as a “stealth” subvariant, does not have the same
missing target gene. Instead, scientists are monitoring it the same way they have
prior variants, including delta, by tracking the number of virus genomes
submitted to public databases such as GISAID.
As with other
variants, an infection with ba.2 can be detected by coronavirus home tests
kits, though they cannot indicate which variant is responsible, experts said.
MORE
TRANSMISSIBLE?
Some early
reports indicate that ba.2 may be even more infectious than the already
extremely contagious ba.1, but there is no evidence so far that it is more
likely to evade vaccine protection.
Danish health
officials estimate that ba.2 may be 1.5 times more transmissible than BA.1,
based on preliminary data, though it likely does not cause more severe disease.
In England,
a preliminary analysis of contact tracing from Dec 27, 2021, through Jan. 11,
2024, by the UK Health Security Agency (HSA) suggests that household
transmission is higher among contacts of people infected with ba.2 (13.4
percent) compared with other omicron cases (10.3 percent).
The HSA
found no evidence of a difference in vaccine effectiveness, according to the
Jan. 28 report.
A critical
question is whether people who were infected in the ba.1 wave will be protected
from ba.2, said Dr Egon Ozer, an infectious disease expert at Northwestern
University Feinberg School of Medicine in Chicago.
That has
been a concern in Denmark, where some places that saw high case counts of ba.1
infections were reporting rising cases of ba.2, Ozer said.
If prior ba.1
infection does not protect against ba.2, “this could be sort of a
two-humped camel kind of wave,” Ozer said. “It’s too early to know if
that will happen.”
The good
news, he said, is that vaccines and boosters still “keep people out of the
hospital and keep people from dying.”
