The COVID-19 pandemic saw the rise of a new class of vaccines: ones that use mRNA to teach our immune systems how to fend off the virus. Since then, billions of mRNA-based vaccine doses have been given worldwide, boosting protection against COVID-19. However, for a few, those vaccines may trigger allergy-like reactions ranging from mild rash to potentially life-threatening breathing difficulties.
Anyone with such reactions after an initial mRNA shot is generally ineligible for further doses, explains Yun Shan Goh, a Senior Research Scientist from the A*STAR Infectious Diseases Labs (ID Labs). Instead, they’re recommended non-mRNA-based alternatives for booster doses. These include inactivated virus vaccines like CoronaVac, which train the immune system to recognise an infection by exposing it to inert forms of the virus. This differs from mRNA vaccines, which show the immune system how to make copies of viral surface proteins to practice on.
While inactivated vaccines have been used for centuries, Goh and colleagues wanted to test if CoronaVac was an effective alternative to mRNA COVID-19 vaccine boosters. The team, in collaboration with clinicians from various Singaporean hospitals, recruited 105 participants with a history of allergic reactions to such vaccines. They treated some participants with a CoronaVac booster and then tracked their immune responses through antibody, memory B cell and T cell levels.
Compared to the control group receiving mRNA boosters, participants who received the CoronaVac booster produced lower levels of antibodies against the virus’s spike protein. However, to the researchers’ surprise, the levels of antibody-producing memory B cells and virus-killing T cells were comparable between both groups.
“There was no direct evidence that Coronavac’s mechanism of action reduces the risk of such reactions,” said Goh. “However, the data showed that for individuals with limited vaccine options due to excessive allergic reactions to mRNA vaccines, CoronaVac can be an alternative.”
Unfortunately, new SARS-CoV-2 variants such as the Omicron variant pose an added challenge. Goh’s team found that neither mRNA nor CoronaVac boosters offered sufficiently protective immune responses against Omicron.
“Variant-specific immunity remains low following CoronaVac vaccination, similar to mRNA vaccination,” said Goh, adding that additional boosters may be required to shield against Omicron and its subvariants. “Apart from CoronaVac, other non-mRNA vaccines such as the protein-based NVX-CoV2373 should be investigated as an alternative for those with allergic reactions.”
The team is currently building upon their findings by examining how long immune responses to CoronaVac vaccines last and whether additional boosters could extend the vaccine’s protective effects, especially against new variants.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Infectious Diseases Labs (ID Labs), the Singapore Immunology Network (SIgN) and the Bioinformatics Institute (BII).