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National Center for Complementary and Integrative Health: “5 Tips: Natural Products for the Flu and Colds: What Does the Science Say?”
CDC: “Vaccine Effectiveness - How Well Does the Flu Vaccine Work?” “Show Me the Science - When & How to Use Hand Sanitizer,” “Preventing Seasonal Flu Illness,” “CDC Says ‘Take 3’ Actions To Fight The Flu.”
Mayo Clinic: “Vitamin C: Can it prevent colds?”
National Institutes of Health Office of Dietary Supplements: “Vitamin C.”
UpToDate: "The common cold in adults: Treatment and Prevention."