first known sequence of ba.2 was in a sample from 15 november 2021. as of 17 january 2022, ba.2 had been detected in at least 40 countries and in all continents except antarctica. by 31 january, it had been detected in at least 57 countries. in global samples collected from 4 february to 5 march and uploaded to gisaid, ba.2 accounted for c. 34%, compared to 41% for ba.1.1, 25% for ba.1 and less than 1% for ba.3. in a review two weeks later, covering 16 february to 17 march, ba.2 had become the most frequent. however, the data is geographically skewed due to sequencing rate and speed; for example, among the c. 205,000 covid-19 sequences from march that had been uploaded to gisaid as of 22 march, united kingdom and denmark accounted for more than 3⁄4, and most of the remaining were from other european countries, australia, canada and the united states (altogether, c. 6,000 were from africa, asia and latin america). based on gisaid uploads, ba.1 peaked in early january 2022, after which it was overtaken by both ba.1.1 and ba.2. in north america, parts of europe and parts of asia, ba.1 was first outcompeted by ba.1.1. for example, in the united states, france and japan, ba.1.1 became the dominant subvariant in january 2022. by late december 2021/early january 2022, ba.2 appeared to have become dominant in at least parts of india (already making up almost 80 percent in kolkata in late december 2021) and the philippines, had become frequent in scandinavia, south africa and singapore, and was showing signs of growth in germany and the united kingdom. in japan, which has quarantine and detailed screening of all international travellers, as of 24 january, the vast majority of ba.2 had been detected in people that had arrived from india or the philippines with cases going back at least to 1 december 2021 (far fewer ba.1 or other variants were detected among arrivals from the two countries in that period), but small numbers had also been detected in people arriving from other countries. in denmark, the first ba.2 was in a sample collected on 5 december 2021 and extremely few were found in the directly following period. by week fifty (13–19 december) it had started to increase, with ba.2 being at around 2 percent of sequenced cases compared to 46 percent ba.1 (remaining delta). the frequency of both omicron subvariants continued to increase throughout the last half of december; in week fifty-two (27 december–2 january), ba.2 had reached 20 percent and ba.1 peaked at 72 percent. in january 2022, ba.1 began decreasing, whereas ba.2 continued its increase. by the second week (10–16 january) of 2022, the frequency of the two was almost equal, both being near 50 percent (around one percent was the rapidly disappearing delta). in the following week, ba.2 became clearly dominant in denmark with 65 percent of new cases being the ba.2 subvariant. trends from the other scandinavian countries, india, south africa and the united kingdom also showed that ba.2 was increasing in proportion to ba.1. in early february 2022, it had become the dominant subvariant in south africa, in late february it had become dominant in germany and in early march it had become dominant in the united kingdom. in early march, ba.1.1 was still heavily dominant in the united states (having overtaken ba.1 in january), but ba.2 was increasing in frequency, later becoming dominant in the us by 29 march. ba.2.12 there were two new ba.2 subvariants detected in the us state of new york, which are ba.2.12 (or b.1.1.529.2.12) and ba.2.12.1 (or b.1.1.529.2.12.1), both of which have a significant growth advantage of 23–27% over ba.2 and contributing to a rise in infections in central new york, centred on syracuse and lake ontario, which later became dominant by may 24 in the us. ba.2.75 and ba.2.75.2 the subvariant ba.2.75 (or b.1.1.529.2.75, nicknamed centaurus by the media), first detected in india in may 2022, has been classified as variant under monitoring by the who. additional newer mutations in this line (like ba.2.75.2 aka b.1.1.529.2.75.2 or chiron) may be capable of escaping neutralizing antibodies. xbb and xbb.1 xbb, a recombinant of the ba.2.10.1 and ba.2.75 sublineages, is an omicron subvariant first detected in august 2022. on 20 october 2022, the chief scientist of the world health organization (who), soumya swaminathan, warned that the xbb subvariant of omicron may cause infections in some countries while the severity of the new variants is not yet known. on 9 january 2023 the european cdc said there was suggestive evidence the xbb.1.5 variant had a growth advantage; after becoming dominant in the us, it might become dominant in europe in the following months. from preliminary evidence, they had assessed the xbb variants had no effect on disease severity and transmissibility. in early march 2023, xbb.1.16 first appeared in india and caused a surge of hospitalizations. it was named arcturus on 13 march 2023. ba.3 the third omicron sublineage, ba.3, is very rare. it has the same s-gene target failure (sgtf) deletion (δ69-70) as ba.1. xe a new ba.1–ba.2 recombinant isolated from the uk in january 2022, dubbed the "xe" recombinant, was found by the who to be potentially 10% more transmissible than ba.2, making it about 43% to 76% to more transmissible than ba.1, and making the xe recombinant the most contagious variant identified. ba.4 and ba.5 in april 2022, the who announced it was tracking the ba.4 and ba.5 subvariants with ba.4 having been detected in south africa, botswana, denmark, scotland and england. early indications from data collected in south africa suggested ba.4 and ba.5 have a significant growth advantage over ba.2, which by 12 may earned the status variant of concern by the european centre for disease prevention and control, and, by 20 may, by the uk health security agency. ba.5 was dominant in portugal by 25 may, accounting for two-thirds of all new cases there. by 24 june, ba.4 and ba.5 together had become dominant variants in the uk and germany. these two subvariants became dominant in the united states by 28 june. by late june, ba.5 became the dominant subvariant in france, with 59% of new cases linked to it. on 10 may 2022, a case of a new subvariant ba.5.2.1 was reported in california. on 10 july, the city of shanghai reported its first case of ba.5.2.1, in a man who had flown in from uganda, sparking a new wave of testing. on 22 july, the province of ontario, canada announced that subvariant ba.5.2.1 overtook ba.2.12.1 as the main variant in circulation in ontario around 2 july. regeneron is reporting that ba.5.2.1 is the main variant in australia, belgium, brunei, greece and iceland. the government of canada also reported in late june and july, of the travelers arriving by air who test positive for covid-19, a substantial proportion were ba.5.2.1. bf.7 is a shortened version of the sub-variants full name, which is ba.5.2.1.7. this sub-variant is part of omicron's ba.5 variant, which had the highest number of reported cases globally, accounting for 76.2% of all cases. bq.1 and bq.1.1 since october 2022, two ba.5 subvariants were found: bq.1 (or b.1.1.529.5.3.1.1.1.1.1) and bq.1.1 (or b.1.1.529.5.3.1.1.1.1.1.1). they have been more prevalent in france than any other european country. as of 17 november, 93% of sequences in france were omicron sub-lineage ba.5 and among the ba.5 sub-lineages, bq.1.1 continued to rise (32% vs 25% in the prior week). according to nextclade even 55%. they were reported in the us to have become dominant, accounting for 44% of new infections, up from 33% the previous week. early laboratory tests found that these subvariants were better at escaping first and booster vaccines than previous variants; however, no new treatments appeared to be in development. transmission false-color transmission electron micrograph of an omicron variant coronavirus, shown in pink, replicating within the cytoplasm of an infected vero cell in humans in january 2022, william schaffner, professor of infectious diseases at vanderbilt university medical center, compared the contagiousness of the omicron variant to that of the measles. vaccinated it was not known in november 2021 how omicron would spread in populations with high levels of immunity or if it causes a milder or more severe disease. on 15 december 2021, jenny harries, head of the uk health security agency, told a parliamentary committee that the doubling time of covid-19 in most regions of the uk was now less than two days despite the country's high vaccination rate. she said that the omicron variant of covid-19 is "probably the most significant threat since the start of the pandemic", and that the number of cases in the next few days would be "quite staggering compared to the rate of growth that we've seen in cases for previous variants". natural immunity relating to naturally acquired immunity, anne von gottberg, an expert at the south african national institute for communicable diseases, believed at the beginning of december 2021 that immunity granted by previous variants would not protect against omicron. vaccinated or natural immunity a study suggests that mutations that promote breakthrough infections or antibody-resistance "like those in omicron" could be a new mechanism for viral evolution success of sars-cov-2 and that such may become a dominating mechanism of its evolution. a preprint supports such an explanation of omicron's spread, suggesting that it "primarily can be ascribed to the immune evasiveness rather than an inherent increase in the basic transmissibility". studies showed the variant to escape the majority of existing sars-cov-2 neutralizing antibodies, including those in sera from vaccinated and convalescent individuals. nevertheless, current vaccines are expected to protect against severe illness, hospitalizations, and deaths due to omicron and, on an individual level, the omicron variant is milder than earlier variants that evolved when the antibody/vaccination share was lower than it was when omicron emerged. in contrast to other investigated variants, omicron showed substantial, population-level, evasion of immunity from prior infection as well as a higher ability to evade immunity induced by vaccines. in non-human animals in february 2022, the first confirmed case infecting a wild animal was confirmed by researchers at pennsylvania state university in white-tailed deer in staten island, n.y. surfaces although transmission via fomites is rare, preliminary data indicate that the variant lasts for 194 hours on plastic surfaces and 21 hours on skin, compared with just 56 and 7 hours, respectively, for the original strain. vaccine effectiveness pfizer-biontech (bnt162b2) and moderna (mrna-1273) mrna vaccines provide reduced protection against asymptomatic disease but do reduce the risk of serious illness. on 22 december 2021, the imperial college covid-19 response team reported an about 41% (95% ci, 37–45%) lower risk of a hospitalization requiring a stay of at least 1 night compared to the delta variant, and that the data suggested that recipients of 2 doses of the pfizer–biontech, the moderna or the oxford–astrazeneca vaccine were substantially protected from hospitalization. in january 2022, results from israel suggested that a fourth dose is only partially effective against omicron. many cases of infection broke through, albeit "a bit less than in the control group", even though trial participants had higher antibody levels after the fourth dose. on 23 december 2021, nature indicates that, though omicron likely weakens vaccine protection, reasonable effectiveness against omicron may be maintained with currently available vaccination and boosting approaches. in december, studies, some of which using large nationwide datasets from either israel and denmark, found that vaccine effectiveness of multiple common two-dosed covid-19 vaccines is substantially lower against the omicron variant than for other common variants including the delta variant, and that a new (often a third) dose – a booster dose – is needed and effective, as it substantially reduces deaths from the disease compared to cohorts who received no booster but two doses. vaccines continue to be recommended for omicron and its subvariants. professor paul morgan, immunologist at cardiff university said, "i think a blunting rather than a complete loss is the most likely outcome. the virus can't possibly lose every single epitope on its surface, because if it did that spike protein couldn't work any more. so, while some of the antibodies and t cell clones made against earlier versions of the virus, or against the vaccines may not be effective, there will be others, which will remain effective. 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