evidence: - Safer for people with weak immune systems. [...] - The seasonal flu vaccine is reformulated each year to target the four strains expected to be most common and most dangerous. [...] Everyone should get all recommended vaccines at the recommended times. It is especially important for children and adolescents to get catch-up doses of any missed vaccines or vaccine doses as soon as they can. Adults should get all recommended vaccines for their age or other risk factors such as health condition or occupation. All adults should get tetanus boosters, seasonal flu and COVID-19 vaccines, and any vaccines missed in childhood.