NEW DELHI: Depression, stress and loneliness can weaken the body's immune system, and lower the effectiveness of certain vaccines, including the new COVID-19 preventives that are in development and the early stages of global distribution, scientists say. According to a report accepted for publication in the journal Perspectives on Psychological Science, simple interventions, including exercise and getting a good night's sleep in the 24 hours before vaccination, may maximise the vaccine's initial effectiveness. The researchers noted that even though rigorous testing has shown that the COVID-19 vaccines approved for distribution in the US are highly effective at producing a robust immune response, not everyone will immediately gain their full benefit.