3rd COVID vaccine significantly reduced hospitalizations and deaths during Australia's 1st Omicron wave

The most recently detected severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant of concern (VOC) is the Omicron variant, which was first detected in South Africa on November 9, 2022, after which it rapidly spread throughout the world. The Omicron variant is capable of escaping both infection- and vaccine-induced immunity due to several mutations in its spike protein.

Study: Relative Effectiveness of COVID-19 Vaccination with 3 Compared to 2 Doses Against SARS-CoV-2 B.1.1.529 (Omicron) Among an Australian Population with Low Prior Rates of SARS-CoV-2 Infection. Image Credit: Golden Dayz / Shutterstock.com

Background

The SARS-CoV-2 Omicron variant was first detected in Australia on November 23, 2022. Since Australia had maintained strict border control and quarantine measures from the beginning of the pandemic through November 1, 2022, most of its population was infection naive by the time Omicron had arrived.

Despite low infection rates, Australia achieved a very high vaccination rate against the coronavirus disease 2019 (COVID-19), with about 85.5% of people over the age of 16 years who had received at least two doses of the vaccine by the time Omicron was first detected in this nation.

A significant rise in COVID-19 cases in Australia in December 2021 led to the approval of a third booster dose for those who were 16 years of age or above. Nevertheless, daily COVID-19 cases in Australia were reported to be the highest in January 2022 than at any other time since the start of the pandemic.

In a recent The Lancet preprint, researchers aim to compare and estimate relative vaccine effectiveness (rVE) in people who received three doses of the COVID-19 vaccine as compared to those who received two vaccine doses. The current study also analyzed the impact of previous infection on VE in an almost infection naive population.

About the study

The current study included individuals who were 40 years or older and resided in the Greater Sydney Area. Demographic information, as well as information on vaccination status, previous SARS-CoV-2 infection, hospitalization, and deaths were also collected.

COVID-19 diagnosis was made either by reverse transcription-polymerase chain reaction (RT-PCR) assay or rapid antigen test (RAT). The notification for SARS-CoV-2 infection, along with the method of diagnosis and death, was recorded in the Notifiable Conditions Information Management System (NCIMS).

The researchers then examined two outcomes including infection with SARS-CoV-2 based on the NCIMS notification, as well as SARS-CoV-2-related hospitalization or death.

Study findings

The average age of the participants was 59 years, with 51.3% of the participants being female. Moreover, 75.6% of the study participants were above the median socioeconomic index of Australia, whereas 11% of study participants reported at least one comorbidity.

Individuals who received three vaccine doses were more likely to be older than those who received two doses, have a comorbidity, have a higher socioeconomic status, and were mostly women.

Taken together, over 52% of individuals who received two vaccine doses received the Pfizer-BioNTech BNT162b2 vaccine, whereas 46.8% received the AstraZeneca ChAdOx1 vaccine. Furthermore, 85.4% of individuals who were administered a third booster vaccine received the BNT162b2 vaccine and 13.7 % received the Moderna mRNA-1273 vaccine.

The risk of infection increased three to six months, as well as after six months, following dose two; however, the risk of infection after dose three was moderately low. The rVE against infection waned after three months for two-dose recipients, while it was preserved for six months in individuals who received three vaccine doses.

The rVE of a third vaccine dose was better as compared to two doses against severe disease, along with hospitalization and death rates during the first eight to 89 days. Moreover, the rVE of dose three against dose two in regards to deaths and hospitalizations was lower for individuals who were between 40 to 64 years of age as compared to individuals who were 65 years or older, as well as those with comorbidities.

No difference in vaccine effectiveness was observed among individuals who were between 50 to 69 years of age and received either BNT162b2 or ChAdOx1 vaccine as their primary vaccination course.

The adjusted hazard ratio (aHR) for infection and severe disease was lower for individuals who had a previous infection, along with those who received two vaccine doses, as compared to infection naive individuals who received two vaccine doses.

Conclusions

The current study demonstrated that the protection conferred by vaccines wanes several months after receiving a second dose; however, it can be restored by a third booster dose during the Omicron period. A third vaccine dose could also prevent severe disease, hospitalizations, and deaths due to COVID-19.

Individuals previously infected with the SARS-CoV-2 Delta variant who had also received two vaccine doses were better protected against the Omicron variant as compared to uninfected and vaccinated individuals. Further analysis of vaccine effectiveness is needed to account for the different characteristics of each population, evolving immunity due to infection with different SARS-CoV-2 variants, and different vaccine doses.

Limitations

The current study appears to be biased towards those who are more likely to seek medical care and be vaccinated. A second limitation of the current study is that the confirmation of severe disease using hospitalizations can be biased if the disease is contracted due to hospitalization for some other conditions.

*Important notice

Preprints with The Lancet / SSRN First Look publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Written by

Suchandrima Bhowmik

Suchandrima has a Bachelor of Science (B.Sc.) degree in Microbiology and a Master of Science (M.Sc.) degree in Microbiology from the University of Calcutta, India. The study of health and diseases was always very important to her. In addition to Microbiology, she also gained extensive knowledge in Biochemistry, Immunology, Medical Microbiology, Metabolism, and Biotechnology as part of her master's degree.