IJCS | Volume 33, Nº3, May / June 2020

285 Physical training is also known to improve immune response. Despite the concerns of some athletes about the side effects of vaccination, high exercise intensity and frequency enhance vaccine-responses in elite athletes. 17 This is also true in older people where immunoglobulins concentrations after vaccinations are greater in cardiovascular-trained individuals than in control ones. 17-22 Despite the promising effects of exercise training on the immune system, exercise stress may be associated with an increased risk for upper respiratory tract infection. 23 Acute bouts of heavy exercise and chronic intensive exercise, as usually performed by long-distance runners, compromise host defenses and increase the incidence of upper respiratory tract infections. 24,25 In athletes under heavy training both innate and acquired immunity are often observed to decrease, typically 15-25%. 26 It is true that exercise can modulate many immune system components, altering the susceptibility to infections, which means that exercise training may increase or reduce the susceptibility to infections. Although it may seem paradoxical, this assertive is true, and quite straightforward. A large study 27 about the Hong Kong flu outbreak in 1998 concluded that mild to moderate exercise, performed three to five times per week reduced the risk of mortality, while people who did not exercise or who exercised too much were at greatest risk of death. Thus, the missing piece of the survival puzzle during pandemic is not exercise itself, but its detailed prescription. Exercise prescription during the COVID pandemic Physicians learn how to prescribe various drugs. Although exercise is beneficial for the treatment of several diseases, exercise prescription is not taught to medicine students or residents. 28,29 As any other medication, exercise training needs the right dosing to achieve the desired effects. The American College of Sports Medicine has introduced the FITT-VP principle as a mnemonic with all the points that must me described in exercise prescription: Frequency, Intensity, Type, Time, Volume and Progression. 6 Social restrictions, correctly imposed during the COVID-19 pandemic, is certainly disrupting routine daily activities of people globally. 30 Still, there are exercises that can be safely done at home. Considering the previous discussion on the minimal training volume 4 for health, recommended by the WHO, and the embedded risks of immunity reduction due to high intensity training, the following principles can guide home-based exercise while at social isolation. First do no harm This principle of the Hippocratic Oath applies here. Exercise has its risks, which are not easily gathered remotely. Before starting exercising by themselves, patients must be sure that all questions in the PAR-Q questionnaire 31-33 are negative. In addition, it is not recommended to exercise if the patient is experiencing flu symptoms, sore throat, body aches, shortness of breath, fatigue cough or fever. Patients with any positive answer to the PAR-Q questionnaire or presenting any of these symptoms should seek medical advice before start exercising. Frequency Guidelines suggest that sedentary people start with aerobic exercise 3 to 5 times per week and that resistance training is done 2 to 3 times per week. Flexibility and mobility exercises can be done on most days. Intensity During the pandemics, one should not engage in high intensity exercise. That being said, it is important to know how to evaluate exercise intensity. There are various ways to do that, but some, such as anaerobic threshold, maximal oxygen volume uptake and real maximal heart rate, require exercise tests not available during the pandemic. Although the maximumheart rate calculation (Maximum heart rate=220 -age) presents errors, 34 it is probably the most feasible way to analyze one´s maximal heart rate when maximal exercise tests cannot be done. During light intensity exercise, heart rate is kept below 45% of maximumheart rate. The heart rate during moderate intensity exercise is kept between 64 and 76%of maximumheart rate. Thus, during the pandemic, exercise heart rate should not be higher than 77% of maximum heart rate (vigorous exercise). Resistance exercise prescription usually considers percentages of repetition maximum for each exercise. 6,35 . As repetition maximum tests uses equipment that are not available at home, some adaptations need to be done. Strength exercises that Castro et al. Exercise training fights pandemics Int J Cardiovasc Sci. 2020; 33(3):284-287 Viewpoint