In a recent study published in Nature Communications, researchers analyzed nationwide vaccination data and cycle cut-off data from four laboratories in Israel that perform quantitative reverse transcription-polymerase chain reaction (qRT-PCR). of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). tests to determine the effect of vaccination-induced immunity on the viral loads of the Omicron variant of SARS-CoV-2.
Study: Viral load dynamics of SARS-CoV-2 Delta and Omicron variants after multiple vaccine doses and prior infection. Image Credit: MedMoMedia/Shutterstock
Background
Vaccine efficacy testing typically involves measuring neutralizing antibody titers and examining the protective effects of immune responses against infection and severe symptoms. The transmissibility of the virus as determined by the viral load is often overlooked. However, viral load has important implications for public health initiatives. Cycle threshold values from qRT-PCR tests, which negatively correlate with viral load, can be used to determine the transmissibility and infectivity of the viral variant.
Despite full vaccination and booster regimens among a large portion of the population, Israel experienced a resurgence of the Omicron variant of SARS-CoV-2, prompting a fourth booster dose to be administered to health care workers. , the elderly population and people at high risk. However, while studies have examined the impact of vaccination-induced immunity on Delta-variant viral loads, the efficacy of vaccines in reducing Omicron-variant viral loads remains unexplored.
About the study
In the present study, the researchers performed a retrospective analysis using vaccination data and records of positive SARS-CoV-2 PCR tests, including cycle threshold values from four major Israel Health Maintenance Organization laboratories. Multiple positive tests for the same individual within 90 days were considered a single infection.
Demographic information was combined with encrypted PCR test results and vaccination data to provide each patient’s vaccination status, cycle threshold value, age, and gender. Patients were then grouped according to vaccination status during the infection event, with vaccination status during infection being defined as unvaccinated, two doses with an infection within 10–39, 40–69, or more than 70 days later. from the second dose, three doses with an infection within 10–39, 40–69, or more than 70 days after the third dose, four doses, and naïve with prior SARS-CoV-2 infection.
The follow-up period was also divided based on the dominant variant of SARS-CoV-2, with June 15 to December 1, 2021 being the domain period for the Delta variant, and December 28, 2021 to December 29, 2021. January 2022. being the period of Omicron domination.
Multivariate linear regression analysis was performed for cycle threshold values with vaccination status, age, sex, calendar time, and laboratory used as covariates. Analysis was also performed separately for PCR assays that amplified the SARS-CoV-2 nucleocapsid gene and those that measured the envelope gene.
Results
The results reported that although the vaccines reduced viral loads of the Omicron variant in the short term, the effect of the vaccine on cycle thresholds was not durable. In contrast, immunity induced by previous SARS-CoV-2 infections was found to wane less slowly.
During the dominance of the Delta variant, two doses of the vaccine were observed to produce a 3-fold decrease in viral loads compared with unvaccinated patients, but the efficacy of the second dose was observed to decrease by day 70, with cycle threshold values of patients with two doses being the same as those of unvaccinated patients. The first booster dose (third vaccine) showed a similar trend, with cycle threshold values high in the short term but decreasing towards day 70.
Cycle threshold values for PCR testing during the Omicron period were observed to increase in response to a recent third dose of vaccine, but were not significantly different for patients in the unvaccinated, two-dose, and 2-dose groups. three late doses. The results indicated that the effect of vaccination-induced immunity was less for the Omicron variant than for the Delta variant.
The analysis was performed separately for elderly patients who received the fourth dose of vaccine. The results revealed an increase in cycle threshold values for these patients, with values comparable to those of patients with previous SARS-CoV-2 infections and significantly higher than those of unvaccinated patients. The fourth dose was considered effective in the short term in reducing the viral load.
conclusions
To summarize, the study compared vaccination status with positive SARS-CoV-2 PCR test cycle thresholds of patients who had an infection during the Delta and Omicron variant domain.
The results indicated that the immune effect of the vaccine on viral loads decreased by day 70 for the primary and booster doses. In comparison, immunity induced by previous SARS-CoV-2 infections was observed to wane significantly more slowly. A fourth booster dose showed significant protective effects and decreased viral load, with cycle threshold values comparable to those in patients with prior infections.
While vaccines have been tremendously effective in reducing morbidity and mortality, the waning effect in reducing the transmissibility of the virus calls for a reassessment of vaccination campaigns.
Magazine reference:
Woodbridge, Y. et al. (2022) “Viral load dynamics of SARS-CoV-2 Delta and Omicron variants after multiple vaccine doses and prior infection”, Nature Communications, 13(1). doi: 10.1038/s41467-022-33096-0.
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