The emergence of Omicron, the most recent variant of concern (VOC) of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), fueled concerns about vaccine efficacy and the ability of immunity induced by prior infection to protect against severe outcomes from Omicron infection. A new paper discusses the level of protection offered by vaccination vs. prior infection against Omicron infection, hospitalization, and oxygen requirement.
Omicron, or B.1.1.529, was designated a VOC by the World Health Organization (WHO) primarily because of the high transmissibility, which led to the rapid spread of the virus throughout the world. This was mediated to a large extent by the presence of many mutations, far more than any previous variant of the virus.
These mutations were mostly in the viral spike protein responsible for virus-host cell attachment and entry. These mutations appeared to change the antibody binding sites, or epitopes, of the spike and other viral proteins such that the virus successfully evaded the host immune response.
The current study, published online in the Journal of Infectious Diseases, aimed to understand how far this reduced vaccine efficacy and the protection conferred by prior infection in terms of Omicron infection, hospitalization, and oxygen supplementation. It was carried out in the Czech Republic, which has been offering the messenger ribonucleic acid (mRNA) Pfizer vaccine, the Moderna mRNA vaccine, and the viral vector vaccines from AstraZeneca and Johnson & Johnson.
From December 27, 2020, to February 13, 2021, almost 70% of the population had received double vaccination, and over a third (almost 40%) had taken a third booster dose. Omicron began to be detected at the end of November 2021, becoming dominant early in 2022, with a large number of reinfections and breakthrough infections.
What did the study show?
Using data from the Czech National Information System of Infectious Diseases (ISID), including all those who ever tested positive for this disease, the researchers found that the risk of new infection with Omicron was 43% less within two months of double vaccination compared to unvaccinated controls. This fell to less than 10% after this period.
A booster dose increased protection levels against Omicron to 56% within two months of the third dose, but they fell to 21% shortly thereafter. In contrast, protection against the Delta VOC was at 73% within two months of the second vaccine dose, falling to 57% afterward. Similarly, post-booster dose protection against Delta infection was 90% and 82% within and after two months of the vaccine.
Among those with a history of prior infection, 68% were immune to reinfection after two and within six months of the infection, but only 13% after this period, with Omicron, compared to 95% and 83% correspondingly for Delta infection. Infections more than six months before this may be expected to have been caused by the ancestral D614G or Alpha variants that were highly prevalent before the Delta VOC emerged.
While any combination of vaccination and prior infection protected against 95% of infections with Delta, this occurred only for recent Omicron infections. Hybrid immunity against the Omicron variant waned over time but remained higher than after either infection or vaccination alone. Against the latter variant, the highest level of protection was observed with vaccination followed by infection, but this was not obvious with the Delta variant.
Prior infection without vaccination protected against ~70% of Omicron for up to 6 months, but ~50% from 7-10 months. Thereafter, it reduced Omicron infections by a third for 11-14 months, and only 17% from 14 months onwards. With a prior Delta infection, reinfection rates fell to over 90% for the first 10 months, waning thereafter slightly to 86% and ~80% at up to 14 months and thereafter.
The same pattern was seen, albeit at a higher level, with immunity-conferred protection against hospitalization or oxygen supplementation requirements. The booster dose protected against 80% of hospitalizations and reduced the need for oxygen therapy by 90%, with a similar reduction in the number requiring intensive care, all with Omicron infection. In fact, hybrid immunity by any order of events produced almost the same high level of protection against Omicron-associated severe outcomes.
Omicron was, in fact, found to be clinically milder than the Delta VOC, the odds for hospitalization being 67% lower, while for intensive care and oxygen therapy, the odds were reduced by 86%. In hospitalized patients, moreover, the odds for oxygen requirement with Omicron were 66% lower compared to Delta and 46% lower for intensive care.
What are the implications?
This study corroborates earlier findings showing the immune escape capability of Omicron from both vaccine- and infection-induced immunity. All the vaccines used had lower efficacy against Omicron compared to Delta. Despite the observed waning of immunity over time, a booster dose elevated immunity with major protection against severe outcomes, remaining more stable over the following months.
More importantly, the study suggests the high protection conferred by hybrid immunity against future surges of coronavirus disease 2019 (COVID-19). This was despite evaluating the proportion of severe disease rather than simply using hospitalization as a single measure of severity. With the reduced clinical severity of the Omicron variant and the protection offered by hybrid immunity, the final toll in terms of intensive care and oxygen therapy requirement appears to be about a quarter of that caused by Delta.
Overall, therefore, “Recent vaccination still brings substantial protection against severe outcome for Omicron.”