Study investigates changing patterns of SARS-CoV-2 infection through Delta and Omicron waves

By Nidhi Saha, BDSReviewed by Aimee MolineuxApr 7 2022

A novel coronavirus emerged at the end of 2019 and was first reported in Wuhan province, China. This outbreak rapidly spread and transpired into the ongoing coronavirus disease 2019 (COVID-19) pandemic.

Vaccination was found to be effective in controlling the spread of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative organism for COVID-19, as well as controlling the severity of the disease.

Background

Previous infection with SARS-CoV-2 also confers a degree of immunity against the acquisition of subsequent infection, just like with vaccines. However, data on the longevity of acquired immunity to SARS-CoV-2 remains unknown. Recently, major concerns have been expressed regarding the waning vaccine-acquired immunity, as well as the immune escape by the more recent variants of concern (VOCs), for example, the Omicron variant.

‘Booster’ vaccination programs have been proposed and were also introduced by several nations in late 2021. Another contributing factor to the diminishing decline in the infection rates or the reemergence of the spread of the virus (besides the waning immunity and immune escape of newer VOCs) is the risk behaviors of people. With the passage of time and after vaccination, many individuals tend to ignore isolation and sanitization protocols. Such behaviors can disqualify the benefits of vaccination programs.

Assessing the effectiveness and importance of acquired immunity is a social issue, as vaccination uptake remains disproportionally lower among socioeconomically deprived populations. Yet, most often than not, unadjusted estimates have been projected for vaccine effectiveness in curtailing the infection.

The study

A new study posted to the medRxiv^* preprint server examined whether SARS-CoV-2 infections in England varied by vaccination status, in cases with a previous COVID-19 positive test report individual, and by neighborhood socioeconomic deprivation, across the Delta and Omicron epidemic waves.

This study derived data from the Combined Intelligence for Population Health Action (CIPHA). This is a population health data resource that supports COVID-19 responses in Cheshire and Merseyside, England.

Data of the following time periods were selected:

1. Delta wave – June 3 to September 1, 2021; wherein, delta accounted for 99% of all infections
2. Delta wave – September 1 to November 27, 2021; until the first case of omicron was confirmed in England
3. Omicron wave – December 13, 2021, to March 1, 2022; most cases were of Omicron

For each period, fatalities within the specified duration were excluded.

Findings

Both the Delta waves saw a higher number of cases compared to the previous phases of the pandemic. Omicron brought about a rampant escalation in positive test results, which increased more than double compared to those during the Delta waves—from new infections as well as reinfections.

The percentage of subsequent infections in the Omicron wave was estimated at 11.4, whereas during the Delta waves, the rate was <1%. Of note, the rate of reinfection was almost double in the most deprived than in the least deprived regions.

Unadjusted associations during the Delta waves revealed that vaccinated individuals had a lower predilection of having a positive COVID-19 test. This effect was presumed to be larger among healthcare professionals (HCPs) – who were fully vaccinated and less likely to have a positive test result.

After adjusting for other demographic and social factors (for the Delta waves) strength of associations was reduced. During the second Delta wave, a stronger protective effect was observed among individuals who received three vaccination doses.

On the other hand, in the Omicron wave, the associations varied between models. The unadjusted model revealed positive associations in people who received one or two vaccination doses and a negative association for three vaccination doses, compared to the unvaccinated individuals. However, HCPs elicited negative associations for all vaccination levels. Thus, vaccinated HCPs were less likely to have a positive test.

Overall, the rates of infection among individuals who had a previous positive test during the three waves and in both models were lower. In the unadjusted models, the effects were larger. Although the unadjusted effect size was smaller during the Omicron wave, it strengthened upon adjustment.

During the first Delta wave, people residing in deprived areas showed higher rates of positive COVID-19 tests, compared to those living in the least deprived areas and the HCPs.

During the second Delta wave, the registered COVID-19 positive tests declined among people residing in deprived areas. Contrastingly, the rates of COVID-19 positive tests were found to be higher among those living in the least deprived areas and the HCPs in this period.

During the Omicron wave, the incidence rates of COVID-19 positive tests were higher among people residing in the least deprived areas, during its initial phase. However, the trends reversed due to the higher peak of infections post-Christmas period.

Of note, during the last phase of the Omicron wave, the trends were reinstated – the rates of COVID-19 positive tests were higher among those living in the least deprived areas. Subsequent infections remained higher in the most deprived regions during the larger part of this wave.

The results were consistent for residents in Cheshire and Merseyside, except when adjusting for the vaccination status. HCPs above the age of 65 years also exhibited similar associations, with minor alterations in vaccine associations during the earlier waves.

Conclusion

It was inferred that socially patterned acquired immunity is rendering complex socioeconomic inequalities. Populations must be made aware of the changing trends of the risks of SARS-CoV-2 exposure and infections.