In a recent study posted to the medRxiv* preprint server, researchers evaluated plasma samples from individuals vaccinated with a fourth booster dose of mono- or bivalent messenger ribonucleic acid (mRNA) severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) vaccine to evaluate the recognition and neutralization capacity against the ancestral (D614G) strain and the Omicron BQ.1.1 subvariant.

Background

The emergent Omicron subvariants with mutations in the spike glycoprotein region challenge the immunity induced by early coronavirus disease 2019 (COVID-19) vaccines. The newly emergent Omicron subvariant BQ.1.1 has shown resistance against the humoral immunity elicited by the monovalent mRNA vaccine and is now one of the major circulating SARS-CoV-2 variants in many countries. The BQ.1.1 subvariant carries five convergent mutations in the receptor binding domain that enhance its ability to escape vaccine-induced immunity.

Health authorities in various countries have now approved the use of bivalent mRNA vaccines, such as those developed by Pfizer and Moderna, which encode the spike proteins of the ancestral SARS-CoV-2 strain and an Omicron subvariant. Furthermore, studies have provided evidence that hybrid immunity, induced by vaccinations and previous SARS-CoV-2 infections, provides better protection against severe infections than vaccine-induced immunity alone.

Furthermore, the waning of vaccine-induced immunity and the emergence of immune-evading Omicron infections has led to an increase in breakthrough infections. Investigating the neutralizing capacity of plasma samples from individuals with vaccinations and breakthrough infections will help understand the protection offered by hybrid immunity against emergent Omicron subvariants such as BQ.1.1.

About the study

In the present study, plasma samples were collected from 63 individuals vaccinated with three doses of the monovalent Pfizer mRNA vaccine and the fourth dose of either the Pfizer or Moderna monovalent mRNA vaccines or the Pfizer or Moderna bivalent vaccines containing BA.4/BA.5 or BA.1 spike protein mRNAs, respectively.

Of the 63 individuals, 20 had confirmed breakthrough Omicron infections with increased anti-nucleocapsid antibodies between the fourth week and fourth month after the third dose or between four months after the third dose and four weeks after the fourth dose of the vaccine. While the remaining 43 individuals might have had previous SARS-CoV-2 infections, no significant increase in anti-nucleocapsid antibodies was detected in their serum samples.

Enzyme-linked immunosorbent assay (ELISA) was used to detect the anti-nucleocapsid immunoglobulin levels in all plasma samples. Human embryonic kidney (HEK) 293T cells transfected with a green fluorescent protein expressor and full-length spikes of SARS-CoV-2 were used for the flow cytometry analysis to detect the recognition of the ancestral and BQ.1.1 spikes by the plasma. Additionally, HEK 293T cells expressing angiotensin-converting enzyme-2 (ACE-2) and lentiviral vectors with plasmids encoding spike proteins of the ancestral D614G strain and the Omicron BQ.1.1 subvariant were used for the virus-neutralization assay. The neutralization at the half-maximal inhibitory concentration (IC₅₀) was calculated for the plasma samples.

Results

The results indicated no significant difference in recognition of the ancestral D614G strain spike protein four weeks and four months after the third mRNA vaccine dose between individuals with and without breakthrough infections. However, compared to individuals without breakthrough Omicron infections, individuals with breakthrough infections showed better recognition of the D614G strain spike protein after the fourth vaccine dose, irrespective of the type of vaccine.

In contrast, individuals with breakthrough infections showed better recognition of the Omicron BQ.1.1 spike protein at four months after the third vaccine dose than those who did not have previous Omicron infections. This difference in BQ.1.1 spike protein recognition was more pronounced four weeks after the fourth vaccine dose. However, the level of spike protein recognition was lower for the BQ.1.1 subvariant than that of the D614G strain.

Four weeks and four months after the third dose, no significant differences were detected in the neutralizing ability of the plasma samples from individuals with and without breakthrough Omicron infections. However, after the fourth vaccine dose, individuals with breakthrough infections developed significantly higher neutralizing antibodies against the ancestral D614G strain and the BQ.1.1 subvariant at four weeks than individuals without breakthrough infections.

All individuals with breakthrough Omicron infections developed neutralizing antibodies against the BQ.1.1 spike protein after the fourth vaccine dose. However, even after the fourth vaccine dose, the neutralization of the BQ.1.1 spike was lower than that of the D614G spike protein. While the results suggested that the Moderna bivalent mRNA vaccine induced better recognition and neutralization of BQ.1.1 spike protein than the Pfizer bivalent vaccine, the results were insignificant.

Conclusions

Overall, the results reported that hybrid immunity induced by breakthrough SARS-CoV-2 Omicron infections and four doses of the mono- or bivalent mRNA vaccine resulted in increased recognition and neutralization of the emergent Omicron BQ.1.1 subvariant.

*Important notice

medRxiv 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.