Researchers at Tel Aviv University (TAU) have demonstrated for the first time that there is a significant link between behavioral stress and the effectiveness of vaccines. They found that acute stress in mice 9-12 days after vaccination increases antibody response to the vaccine by 70% compared to an unstressed control group. But this comes at the price of reduced antibody breadth, which results in diminished protection against the pathogen’s variants.

The study was led by PhD student Noam Ben-Shalom from the lab of Dr. Natalia Freund at TAU’s Faculty of Medicine and PhD student Elad Sandbank from the Neuro-Immunology Lab of Professor Shamgar Ben-Eliyahu at TAU’s School of Psychological Sciences and Sagol School of Neuroscience. The paper was published in the October 2023 issue of Brain, Behavior, and Immunity.

“In this study we examined the correlation between stress and the body’s ability to develop an immune response following vaccination for the first time,” Dr. Freund says. “The prevailing assumption is that the effectiveness of a vaccine is determined mainly by its own quality. However, over the years, professional literature has reported influences of other factors as well, such as the age, genetics, and microbiome of the outcomes of vaccination. Our study was the first to investigate the possible effects of acute stress. We found that this mental state has a dramatic impact on not only the vaccine’s effectiveness but also how it works.”

Acute stress is a mental state caused by immediate threat (either real or imagined), involving the secretion of adrenaline and stimulation. In this study, Dr. Freund and her colleagues vaccinated mice with two different vaccines: the model protein Ovalbumin and a fragment of the SARS-CoV-2 spike protein also used in the COVID-19 vaccine. Nine days later, just as the adaptive immunity became active and the production of antibodies began, the mice were subjected to a widely-used behavioral paradigm simulating acute stress. Thirty days after exposure to stress, the level of antibodies in the blood of vaccinated animals that had experienced stress was 70% higher compared to the control group. This phenomenon was observed in animals vaccinated with either type of vaccine.

At the same time, the researchers discovered that the immune system of the animals that had experienced stress was not cross-reactive to variants of the protein used in the vaccine. In other words, the immune system was focused entirely on the original vaccine following stress, showing no response to proteins that were only slightly different, such as variants of concern (VOC) of SARS-CoV-2.

“Initially, we were surprised to find out that the response to the vaccine was much more effective in animals that had experienced stress,” says Dr. Freund. “We would have assumed just the opposite — that stressful situations would have a negative impact on the immune system. Nevertheless, with both types of vaccines, we observed a stronger immune response after stress, both in the blood and in B cells (the lymphocytes that produce antibodies) derived from the spleen and lymph nodes of the immunized mice.

“The enhancement of the antibodies’ activity following stress was mediated by the beta2 adrenergic receptor that identifies adrenaline. When we blocked this receptor, either pharmacologically or by means of genetic engineering, the effects of stress were completely eliminated.

“On the other hand, to our great surprise, the breadth of the immune response generated by the vaccine was reduced by about 50% following stress. In general, the purpose of vaccination is not only protection against a specific pathogen, but also creating a long-lasting immunological memory for protection against future mutations of that pathogen. In this sense, the vaccines appeared to lose much of their effectiveness after exposure to stress.”

According to the researchers, this is in fact a classical “fight or flight” response demonstrated at the molecular level. During stress, the immune system produces large quantities of antibodies and stronger antibodies to address the immediate infection, and this large energetic investment in the here-and-now comes at the expense of future immunological memory.

“In the second part of the study we wanted to test whether humans also display the post-stress immune impairment observed in vaccinated mice,” Dr. Freund continues. “For this purpose, we cultured B cells obtained from blood of people who had contracted COVID-19 in the first wave. We then induced stress in these cultures using an adrenaline-like substance that stimulates the beta2 adrenergic receptor, that was identified by us in the first part of the study as a mediator of the response to stress in cells that produce antibodies in mice. B cells express a very high level of these receptors, but until now the receptors’ role in producing antibodies was not known. In fact, it was unclear why these cells need the ability to respond to adrenaline.

“We discovered that, just like in mice, human cells also exhibit a zero-sum game between the intensity and breadth of the immune response. When the adrenaline receptor is activated during stress, the entire immune system is stimulated, generating antibodies that are 100 times stronger than antibodies produced in cells that had not undergone stress. But here too, the response was narrower: the diversity of antibodies was reduced by 20-100%, depending on the individual from whom the cells were taken.

“Stress can be caused by different factors,” Dr. Freund concludes. “We tend to think of mental stress, but physical illness also causes a form of stress. When the body contracts a virus or bacteria it experiences stress and signals to the immune system that the top priority is getting rid of the pathogen. Investing energy in long-term immunological memory becomes a second priority. Therefore, stress 9 to 12 days after vaccination, at the time when B cells are generating high affinity antibodies, enhances short-term immunity and damages long-term memory.”