WHO Update on Covid-19: Trends in Cases, Transmissibility, Disease severeness and Vaccine Effectiveness



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Global Overview of Covid -19 as Of 23 January 2022

A slower increase in case incidence was observed at the global level, Globally, the number of new COVID-19 cases increased by 5% in the past week (17-23 January 2022), while the number of new deaths remained similar to that reported during the previous week.. Across the six WHO regions (Americas, Europe, Africa, South-East Asia, Eastern Mediterranean, Western Pacific) over 21 million new cases were reported, representing the highest number of weekly cases recorded since the beginning of the pandemic. Nearly 50 000 new deaths were also reported. As of 23 January2022, over 346 million confirmed cases and over 5.5 million deaths have been reported in total.

At the country level, the highest numbers of new cases were reported from the United States of America (4 215852 new cases; a 24% decrease), France (2 443 821 new cases; a 21% increase), India (2 115 100 new cases; a 33% increase), Italy (1 231 741 new cases; similar to the previous week), and Brazil (824 579 new cases; a 73% increase).

WHO, in collaboration with national authorities, institutions and researchers, routinely assesses if variants of SARS-

CoV-2 alter transmission or disease characteristics, or impact effectiveness of vaccines, therapeutics, diagnostics or

public health and social measures (PHSM) applied to control disease spread. Potential variants of concern (VOCs),variants of interest (VOIs) or variants under monitoring (VUMs) are regularly assessed based on the risk posed toglobal public health. As evidence becomes available, classifications of variants will be revised to reflect thecontinuous evolution of circulating variantsand their changing epidemiology. Criteria for variant classification, andthe current lists of VOCs, VOIs and VUMs, are available on the WHO Tracking SARS-CoV-2 variants website. National authorities may choose to designate other variants of local interest/concern and are encouraged to investigate and report on the impacts of these variants.


Geographic spread and prevalence of VOCs

The current global epidemiology of SARS-CoV-2 is characterized by the dominance of the Omicron variant on a globalscale, continued decline in the prevalence of the Delta variant, and very low-level circulation of Alpha, Beta andGamma variants. The Omicron variant includes Pango lineages B.1.1.529, BA.1, BA.2 and BA.3. BA.1 accounts for98.8% of sequences submitted to GISAID as of 25 January 2022, although a number of countries have reported recentincreases in the proportion of BA.2 sequences. All these variants are being monitored by WHO under the umbrella of ‘Omicron’. Following the identification of travel-related cases of the Omicron variant, many countries are now reporting community transmission. Countries that experienced a rapid rise in Omicron cases in November and December 2021 have been or are beginning to see declines in cases.


Update on the Omicron Varient

Based on the currently available evidence, the overall risk related to the Omicron variant remains very high. Compared to other variants, Omicron has shown an increased ability to spread within the community, leading to a rapid increase in the numbers of new cases in multiple countries where it has replaced other variants, including Delta. Despite this, there appears to be a lower risk of severe disease and death following Omicron infection as compared to other variants. However, due to the very high numbers of cases, many countries have seen a significant increase in the incidence of hospitalization, putting pressure on healthcare systems.



The Omicron variant has a significant growth advantage, a higher secondary attack rate and a higher observed reproduction number as compared to the Delta variant, and as a result, it is rapidly replacing the latter globally. It is thought that this transmission advantage is largely due to Omicron’s ability to evade immunity following infection and/or vaccination. However, compared to the Delta variant, Omicron is able to more rapidly infect the tissues of upper respiratory tract rather than the lungs, which may also help the spread of this variant. Studies conducted in the United Kingdom and Denmark showed that household contacts of cases with the Omicron variant were more likely to be infected as compared to those who were contacts of cases with the Delta variant: the household secondary attack rate in the study conducted in the United Kingdom was 13.6% for the Omicron as compared to 10.1% for the Delta variant; and for the study conducted in Denmark, 31% for the Omicron as compared to 21% for the Delta variant.1 Additionally, studies conducted in India and South Africa have reported a higher proportion of asymptomatic infection at the time of testing among individuals infected with Omicron compared to infection with Delta. The higher occurrence of asymptomatic presentation may result in a lower rate of detection, and thus may further contribute to transmission.


Disease Severity

Epidemiological trends continue to show a decoupling between case incidence, hospital admissions and deaths in most countries, when compared to epidemic waves due to previous variants. This is likely due to a combination of the lower intrinsic severity of the Omicron variant (including increased likelihood of replication in the upper respiratory tract rather than the lungs), and preservation of protection against severe disease following vaccination. Several studies have evaluated the risk of hospitalization and severe disease with Omicron as compared to Delta.

An analysis from the United Kingdom Health Security Agency with the Medical Research Council (MRC) Biostatistics Unit, the University of Cambridge,showed a 47% reduction in the risk of presentation to emergency care or hospital admission with Omicron compared to Delta and 66% reduction in the risk of admission from emergency departments. However, uncertainty remains about the severity and the impact on hospitalizations in populations with low vaccination coverage or prior exposure to SARS-CoV-2 infection.


Impact on Immunity

The Omicron variant has an increased ability to evade immunity as compared to prior variants, causing re-infections in those who have had a previous infection and in those who have been vaccinated. Here we summarise the risk of re-infection as further details on vaccine effectiveness are given in the section on vaccines below. A study conducted in the United Kingdom found that, when compared to the Delta variant, the risk of reinfection with the Omicron variant was 5.4-fold higher.

In those who were unvaccinated, this risk was slightly higher at 6.4-fold and in those

who were vaccinated, slightly lower at 5.0-fold. A separate study conducted in the United Kingdom found that those who had a lower viral load (higher Ct value) during their previous infection were at a higher risk of reinfection.


Studies Related To Vaccine Effectiveness for Omicron VOC and Delta VOC

Omicron Variant

Since the 11 January update, three new studies have provided additional evidence of reduced vaccine effectiveness of mRNA vaccines against infection and symptomatic disease due to the Omicron variant.1–3 These studies report decreased VE of two doses of mRNA vaccines against infection and symptomatic disease due to the Omicron variant compared to the Delta variant within the first few months of receipt of the second dose, with VE estimates decreasing  more rapidly with increasing time from completion of the primary series.

A peer-reviewed study from the United States of America reported VE estimates against symptomatic disease of approximately 40% and 30% for Moderna-mRNA-1273 and Pfizer BioNTech-Comirnaty, respectively, one month following two doses of vaccination. However, by 6-7 months following the second dose, VE had declined to 0% for both vaccines.

A second study of adults in the United States of America (not yet peer reviewed) provides new evidence of the VE of Pfizer BioNTech-Comirnaty against hospitalization due to the Omicron variant.2 The vaccine was 70% effective at preventing hospitalization due to Omicron within the first 3 months of the second dose with no decrease in VE found at 6 months. This same study found a third dose of Pfizer BioNTech-Comirnaty increased VE against hospitalization

due to Omicron to 89% (83-92%) which was sustained at 3-5 months. However, a VE against emergency department visits not leading to hospital admission decreased from 78% (95%CI 73-82%) immediately following the third dose to 48% (95%CI 14-69%) at 6 months or more.

In addition to VE against hospitalization, the three studies also showed that a third dose of mRNA vaccine increased VE from 0% to 62-78% for infection and symptomatic disease in the first 3-5 months following a third dose.

Delta Variant

Five new studies ( four pre-prints and on peer-reviewed study) provided further evidence of performance of two doses of vaccine against the Delta variant.43–47 The VE of the Pfizer BioNtech-Comirnaty vaccine against infection and symptomatic disease within 1-3 months after receipt of the second dose ranged from 80-91% but decreased with increasing time to a VE of 53-79% at four or more months following the second dose. The VE against hospitalization

for Pfizer BioNtech-Comirnaty was high (88-95%) after the second dose. However, one study showed a reduction in the VE against hospitalization to 74% (65-80%) at six or more months following the second dose. Five new studies (three pre-prints and two peer-reviewed studies) also provide further evidence of the vaccine performance of a third dose against infection, symptomatic disease, and hospitalization when the Delta variant was the predominant circulating variant.42–44,48,49 The VE of three doses of mRNA vaccines against infection and

symptomatic disease ranged from 77-96% across studies. Two studies (both not yet peer-reviewed) evaluated VE of three doses of the Pfizer BioNTech-Comirnaty vaccine against hospitalization. The first study reported a relative VE (as compared to those receiving two doses, five or more months prior) of 89% (95%CI 87-

91%).48 The second study reported an absolute VE (as compared to those unvaccinated) of 95% (95%CI 91-97%) against hospitalization due to the Delta variant within three months of receipt of the third dose; however, VE at approximately 3-5 months decreased to 65% (16-85%).



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