Vaccine effectiveness of mRNA-1273 during the emergence of the SARS-CoV-2 Delta variant

By Sam HancockReviewed by Danielle Ellis, B.Sc.Dec 16 2021

Most developed countries have opted for mass vaccination programs to better the current coronavirus disease 2019 (COVID-19) pandemic. The vaccination programs allowed the dismantling of many emergency measures such as lockdowns and social distancing restrictions. However, some studies have suggested that two vaccine shots are insufficient to protect individuals from infection by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Study: Durability of mRNA-1273 against COVID-19 in the time of Delta: Interim results from an observational cohort study. Image Credit: Elzbieta Krzysztof/Shutterstock

A preprint version of the study is available on the medRxiv* server while the article undergoes peer review.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources

The study

The information was gathered from the Kaiser Permanente Southern California (KPSC) integrated healthcare record. Any individual over 18 years old that had been a member for at least a year before the study was eligible for study participation. Individuals who had received a vaccine other than the mRNA-1273 vaccine were excluded, as were those who had previously been infected with COVID-19.  This information was gathered from the electronic healthcare record of the KPSC, including SARS-CoV-2 infection history.

The unvaccinated cohort was made from individuals who had not received any COVID-19 vaccinations. The first outcome the researchers were investigating was SARS-CoV-2 infection, defined by a diagnosis or positive test result. The second outcome they investigated was the severity of the disease, determined by hospitalization or death. They took into account the rise of the Delta strain in their analysis.

Other variables assessed include age, sex, race/ethnicity, socioeconomic data, medical center area, pregnancy, and occupation. Continuous variables were compared using a two-sample t-test or Wilcoxon rank-sum test, while categorical variables were compared using the Xsquared test or Fisher’s exact test. Absolute standardized differences were used to assess the balance of covariates such as race, age, and sex across groups.

Incident events divided by person-years were used to calculate incidence rates for infection, hospitalization, and death. Kaplan-Meier curves showed cumulative incidence for these events. Cox proportional hazards regression models allowed adjusted and unadjusted hazard ratios, and vaccination efficiency was calculated as (1-hazard ratio) x 100. Vaccine efficiency was calculated every two months post-vaccination.

The researchers collected information from nearly one million vaccinated and one million matched unvaccinated individuals. All of the vaccinated individuals had received two doses of the mRNA-1273 vaccine. The median age of these individuals was 52 years, with roughly equal male and female participants. The participants consisted of a range of ethnicities. The separate cohorts were matched based on BMI, smoking, Charlson comorbidities index, frailty index, chronic diseases, immunocompromised status, autoimmune conditions, pregnancy status, history of SARS-CoV-2 infection, emergency department visits, hospitalization, insurance enrolment, and median household income.

There were 7685 infections amongst the vaccinated cohort and 16,809 infections amongst the unvaccinated, leading to an incidence rate (IR) per person of 19.09 and 94.02, respectively. IRs for hospitalization and death were 0.60/0.0 and 13.07/0.90, respectively. As expected, the incidence of infection, hospitalization, and death were higher in unvaccinated than vaccinated individuals, with the vaccine efficiency (VE) calculated at 82.8, 96.8 and 97.2 against infection, hospitalization, and death.

When examining the VE against infection in subgroups, the IR was larger in under 65s than in over 65s. IRs were also higher in women than men and higher amongst Hispanic and Black individuals. The IRs were also significantly higher for both vaccinated and unvaccinated individuals while the Delta variant was dominant, at 20.38 for vaccinated individuals and 146.73 for unvaccinated individuals. The researchers also found that VE against SARS-CoV-2 infection faded over time, from 88.0% between 0 and 2 months to 75.5% at 6-8 months. VE against hospitalization remained stable.

Conclusion

The authors highlight that their research confirms the effectiveness of two doses of mRNA-1273 against SARS-CoV-2 infection. The vaccine efficiency against hospitalization and death remains high and wanes significantly less than the vaccine efficiency against infection. While the efficiency against infection does wane with time, the incidence rates for infection, death, and hospitalization are significantly lower for the vaccinated individuals than the unvaccinated individuals.

However, the researchers approve the recommendations of a booster shot at least six months after the initial two, echoing calls from multiple other scientists. These findings could be essential for public health policymakers and vaccine manufacturers. They could play key roles in helping to model the future spread of the pandemic and the burden likely to be placed on healthcare services.

This news article was a review of a preliminary scientific report that had not undergone peer-review at the time of publication. Since its initial publication, the scientific report has now been peer reviewed and accepted for publication in a Scientific Journal. Links to the preliminary and peer-reviewed reports are available in the Sources section at the bottom of this article. View Sources