Scientists have unlocked most secrets of the Omicron variant (as well as sister lineages such as BA.2), particularly those that are of most real-world consequence: the variant spreads faster than any other before, but is significantly less likely to lead to severe disease. It is the most resistant to past immunity, but vaccines still work in avoiding deaths.
What is not known is how much of these traits are intrinsic to the virus, compared to characteristics that are the consequence of baseline immunity in the global population in what is now the beginning of the third year of the pandemic.
Among the specific unknowns is why the Omicron variant spreads as quickly as it does.
The earliest theory that sought to explain this originated from lab studies that showed the virus to multiply more readily in parts of the respiratory system known as the bronchus, and thrive more widely in upper air passages. This, it was believed, allowed the virus to occupy tissues that give it an advantage in being exhaled or coughed out.
A second theory was that the virus was causing significantly more asymptomatic infections, which meant it spread more readily by people who were simply not aware of their infection.
A third theory, which has been spoken about since the early days of the Omicron outbreak, is that the virus spreads more readily because it can infect more people, even those with past infections or vaccination because of its improved resistance.
{{/usCountry}}A third theory, which has been spoken about since the early days of the Omicron outbreak, is that the virus spreads more readily because it can infect more people, even those with past infections or vaccination because of its improved resistance.
{{/usCountry}}Now, a study from Brazil gives more credence to the third theory.
The case of Amazonas
Researchers from Brazil’s Oswaldo Cruz Foundation, or Fiocruz, found that in the Brazilia state of Amazonas, the Omicron variant triggered a record-breaking wave of infections even when the population had a baseline immunity that successfully stopped the Delta variant from taking hold.
Amazonas, which is home to the city of Manaus – one of the most densely populated places in the world – had two outbreaks in early 2020 and early 2021. Its first outbreak was driven by the B.1 lineage, closest to the virus that spread during the first year of the pandemic around the world, and the second driven by the P.1 variant of concern.
Both outbreaks were large, and most of the population was infected in these waves, and there was also large vaccination coverage. “The high prevalence of individuals with hybrid (natural or vaccine-induced) immunity in this Brazilian state successfully limited the previous expansion of VOCs Gamma plus (P.1.) and Delta (B.1.617.2/AY.) that emerged and spread during the second half of 2021 in the Amazonas,” the team from Fiocruz wrote.
Consequently, the virus circulated at a roughly steady-state level of roughly 50-500 positive cases per day (seven-day rolling average) from early May to late December 2021, they said.
But, in January, “the mean daily number of Sars-CoV-2 positive cases in the Amazonas rapidly increased from 90 to 6,500, coinciding with the global spread of the immune escape VOC (variant of concern) Omicron (B.1.1.529/BA.*),” they added,
In their assessment, the scientists added, the hybrid immunity “however, could not prevent a new expansion of Sars-CoV-2 cases associated with the spread of the Omicron variant that combined both enhanced transmissibility and substantial immune escape”.
In other words, they believe the virus has an intrinsic transmission advantage and also finds new hosts since prior infection is not protection enough.
“Despite the record number of positive cases registered in Amazonas in January 2022, the proportion of deaths during the Omicron wave is currently much lower than that registered during previous epidemic waves or the previous endemic-like period,” they said.
“This observation is consistent with the notion that acquired immunity (natural plus vaccines) effectively reduces the infection fatality rate of Sars-CoV-2 in Amazonas over time,” they added.
It is likely that the immune evasiveness of Omicron is only as strong enough for it to cause reinfections. The immune resistance, it is now clear, is not strong enough for it to trigger more severe disease.
This very trait could also explain the second phenomenon described at the beginning: asymptomatic infections. While asymptomatic infections pose an epidemiological challenge in contact-tracing and isolation, with Omicron, it reflects a positive — the coronavirus in this configuration is now milder, either as a result of baseline immunity or intrinsic evolutionary traits. Or, perhaps, both.