Australian researchers have discovered why messenger ribonucleic acid (mRNA) vaccines can cause side effects such as headaches and fevers in a breakthrough that they said could help improve the efficacy of vaccines.

A new study published by Melbourne's Peter Doherty Institute for Infection and Immunity and RMIT University on Thursday provided the first detailed analysis of how mRNA vaccines circulate and break down in the human bloodstream.

The mRNA vaccines are designed to stay in the lymph nodes to produce antibodies to fight infections, but the research, which analyzed 156 blood samples from 19 individuals over 28 days after receiving an mRNA COVID-19 booster immunization, discovered that a tiny amount of the vaccine found its way into the bloodstream.

"The extent to which the vaccine enters the bloodstream varies between individuals, which may explain some of the side effects such as fever, headache, and fatigue, reported after vaccination," said Yi Ju, co-author of the research from the School of Science at RMIT University.

"This variation in vaccine presence in the blood could trigger inflammatory responses, leading to these side effects in certain individuals."

The first mRNA vaccines were approved for COVID-19 in 2020. Rather than using a weakened virus, mRNA vaccines use genetic instructions to prompt the body to produce a protein that triggers an immune response.

Since their introduction, scientists have used mRNA vaccine technology to develop vaccines and therapeutics for other conditions including cancer.

The authors of the new study said the discovery offered valuable insights into improving mRNA vaccines for safer and more effective use.