Regular physical exercise improves the release of pro- and anti-inflammatory cytokines, lymphocyte circulation, and cell recruitment, which beneficially affects the immune system by reducing the occurrence, severity of symptoms, and death rates in viral diseases such as COVID-19 [52,53]. Strategies to ensure physical activity could include homebased (indoor or confined exercises) and outdoor-based (outdoor exercises) activities. In the case of COVID-19-induced lockdowns and restrictions, indoor exercises will help prevent inactivity, idleness, and frequent bed rest. Additionally, it will promote adherence to exercise routines, which will improve health and prevent the development of other chronic diseases and severe COVID-19-related manifestations [54].

Minimal-intensity aerobic activity at 60%–75% of maximum heart rate, 50%–60% of VO₂ maximum (VO₂max), and 40%–60% heart rate reserve, with a perceived exertion rating of 10–14 out of 20, for 3–5 days per week, and lasting 20–60 minutes per session, can be beneficial to people with chronic conditions such as DM and hypertension. Additionally, it promotes mental health and significantly improves the immune system, which helps prevent the occurrence of COVID-19 [55]. Brisk and treadmill walking are examples of aerobic activities that confer these beneficial effects [55]. Furthermore, minimal-intensity aerobic activities (walking or cycling) for 20–60 minutes, 2–3 times per week, with an intensity of 55%–80% VO₂max or 60%–80% of maximum heart rate, will enhance immune parameters (leukocytes, lymphocytes, neutrophils, monocytes, eosinophils, interleukin [IL]-6, CD16-56, CD16, CD4, CD3, CD8, and CD19) of people with COVID-19 without any adverse effects [56].

The WHO’s advisory of physical exercise requirements (at least 150 minutes of minimal-intensity physical exercise, 75 minutes of high-intensity physical exercise, or an equivalent combination of the 2 weekly) could still be met when staying at home; a combination of five home duties in a week for >5 minutes each daily can be performed [57]. Furthermore, following phone app exercise sessions (Aaptiv, Strava, Peloton, Nike training club, and Zombies Run), television exercise programs, or exercise instructions on the radio will enable adherence to routine physical activities while at home [58].

Adequate diet and nutrition ensure an optimal immune system to prevent infectious illnesses [59]. Suboptimal nutrient levels are related to inflammation and oxidative stress, which affects the body’s immunity [59]. Adequate protein consumption is important for higher antibody secretion, while reduced vitamin A or zinc levels increase the odds of infection [59]. Most of the patients hospitalized with COVID-19 experienced malnutrition and lower levels of vitamins C, D, and B12, selenium, iron, omega-3, and medium- and long-chain fatty acids, indicating the importance of nutritional intervention for the prevention of COVID-19 [60]. Vitamins C, D, and E and phytochemicals (carotenoids and polyphenols) exert higher anti-inflammatory and antioxidant functions in the body, which prevents viral infections [61]. In addition, dietary fiber fermented into short-chain fatty acids exerts beneficial anti-inflammatory functions [61].

The six vitamins recommended by the European Food Safety Authority are (D, A, C, Folate, B6, and B12) and four minerals (zinc, iron, copper, and selenium) are beneficial for optimal immune functioning [62]. Furthermore, optimal consumption of vitamins D, C, and B and iron-rich diets is associated with lower COVID-19 occurrence and death [62]. The supplementation of these essential micronutrients in vulnerable populations and COVID-19-affected individuals in underdeveloped and developing countries is warranted to diminish the risks of death and severe outcomes during this pandemic [63].

Frequent adherence to the Mediterranean diet confers substantial protection against the development of COVID-19 [64]. The high consumption of sugar-flavored foods, saturated fats, and manufactured carbohydrates (Western diet) increases the occurrence of DM, obesity, and hypertension, risk of COVID-19 infection, and adaptive immunity-related severe outcomes [64]. An unhealthy diet and COVID-19-related peripheral inflammation may lead to long-term deleterious health conditions, such as neurodegenerative diseases and dementia, via multiple neuroinflammatory mechanisms in people who have recovered from COVID-19 [65]. Thus, the consumption and access to a healthy diet is imperative and should be a global priority during and after this pandemic [65].

The WHO recommends at least 4 and 5 serving units of natural fruits and fresh vegetables, respectively, in a day to strengthen the immune system and reduce the occurrence of chronic conditions during this pandemic [66]. Furthermore, a combination of unrefined cereal grains (180 g), assortments of beef (excluding red meat), and beans (160 g) was also suggested by the WHO for optimal nutrition during this pandemic [66]. Natural fruit, dietary fiber, and fresh vegetable sources are rich in important nutrients, such as vitamins (A, C, D, B6 and B12) and other essential micronutrients, which are important for optimal immune function and protection against COVID-19 during this pandemic and later [67].

Adequate and high-quality sleep are imperative in promoting mental and physical well-being. Inadequate sleep leads to low performance, limited alertness, and a general diminishment of health quality [68]. Inadequate sleep is a predisposing factor for malignancies, substance abuse, depression, suicidal thoughts, serious inflammation, stroke, immune system deficiencies, and numerous infections and illnesses that result in severe outcomes, such as COVID-19 [69].

The ongoing COVID-19 health crisis has negatively affected the sleep of the majority of people globally, with many reporting manifestations of insomnia [70]. The general public and medical personnel have been primarily affected by sleep problems [70]. Inadequate or intermittent sleep, which is common among medical personnel on shifts, first responders or paramedics, and social workers, has affected the wellbeing, immune function (innate and acquired immunity), proinflammation stimulation, and susceptibility to viral infections, such as SARS-CoV-2, of an individual due to inflammation and hormonal imbalance [71]. Interruption of breathing during sleep (sleep apnea) is an identified risk factor and a major predisposing factor for COVID-19 [71,72]. Moreover, inadequate sleep increases the incidence of common cold in a dose-dependent manner [73]. These findings suggest that a sleep duration of 7 to 9 hours a night (for adults) could improve the effectiveness of an individual’s immune system, resulting in rapid responses against other viral upper respiratory infections, such as COVID-19 [73].

Adequate sleep may modulate fast adaptive immune responses to infection. Spiegel et al. [74] examined the effect of inadequate sleep on the human antibody response to influenza immunization in 25 people; patients who slept for 8 h/night developed double the immunoglobulins and antibodies against the influenza virus within 10 days after immunization than those who slept for 4 h/night did. Prather et al. [75] examined the association between sleep and antibody responses to hepatitis B immunization; inadequate sleep in normal settings negatively affects an individual’s immune response to new antigens, which possibly explains the association between inadequate sleep and increased susceptibility to infectious diseases. Currently, there is no direct study on the effect of sleep on COVID-19 vaccine and immunity. Nonetheless, the findings of these studies indicate that adequate sleep (7–8 h/night) could improve the immunity of people who have been vaccinated against COVID-19 [76].

During this pandemic, body and skin hygiene has been crucial. Bathing regularly, preferably twice a day (morning and night), with antiseptic soap and clean running water and regular washing of clothes with germ-killing detergents may kill SARS-CoV-2 [77].

Regular dental hygiene during this pandemic and later is imperative; people with periodontal disease may be at an increased risk of severe outcomes from COVID-19 than the general population are [78]. A potential relationship between periodontal disease and COVID-19 has been established through two routes. First, the angiotensin-converting enzyme-2 (ACE-2) and CD147 receptors, which is used by SARSCoV-2 to infect new cells, increases appreciably in the presence of periodontal disease, thus possibly promoting a viral infection [78,79]. Second, IL-6 and IL-17 could potentially cause the onset of periodontal diseases and serious COVID-19 infections [78,79].

Chronic alcohol intake is associated with the onset of numerous diseases and may be a risk factor for COVID-19 [80]. It may also worsen psychological and organic conditions, thereby predisposing an individual to behavioral actions that increase the risk of developing and severity of COVID-19 [80]. A minimal quantity of alcohol can affect the immune response by increasing the inflammatory responses in the body [81]. This plays an important role in the development of pancreatitis and alcoholic liver disease and negatively affects several other tissues and organs in the body [81]. Furthermore, anti-inflammatory cytokines are affected by alcohol intake [81]. Acute alcohol consumption interrupts the healthy functioning of the entire immune system (adaptive immunity), which includes cell-mediated and humoral actions, thereby increasing the risk of any bacterial and viral infections, such as SARS-CoV-2, in chronic alcohol abusers [81].

Smoking impairs lung immune activity and increases the risk of infections from other communicable diseases and the subsequent severe health consequences in patients infected with COVID-19 [82]. Smoking is a major contributing factor to the development of COVID-19 [82]. Kashyap et al. [83] determined that chronic smoking is related to higher disease severity and number of deaths in patients hospitalized for COVID-19. Smoking can also impair the ACE-2 receptors used by SARS-CoV-2 to gain entry into the host cell and initiate cytokine problems, which often leads to severe consequences in patients with COVID-19 [83]. All types of smoking produce exhaled smoke, sneezing or coughing, fumes, and aerosols, which may contain SARS-CoV-2, which in turn could contaminate surfaces and surroundings [84]. Thus, smoking tobacco, especially in poorly ventilated places, could transmit SARS-CoV-2 in serious, passive and non smokers [84].

Cannabis use is related to several serious health consequences, such as respiratory/breathing complications (serious cough and air accumulation in the lung tissues), immune system disruption, and a high risk of contracting or spreading viral infections (SARS-CoV-2 and others) [85]. Thus, smoking or cannabis use during the COVID-19 pandemic should be avoided to prevent infection and COVID-19-related complications.

People with COVID-19, obesity, DM, heart diseases, severe obstructive lung disease, cancers, human immunodeficiency virus, hypertension, and other underlying medical conditions are prone to developing severe outcomes (morbidity and mortality) due to the strong affinity of the SARS-CoV-2 to ACE-2 receptor, which is expressed more in patients with comorbidities [86].

Nikoloski et al. [87] determined that people with specific medical conditions, including hypertension, heart diseases, severe liver and kidney diseases, DM, and serious lung conditions, are negatively affected by COVID-19 than people without any medical conditions; SARS-CoV-2 has a strong affinity for the ACE-2 receptor and a hyper-inflammatory response (cytokine storm) is commonly seen in people with comorbidities. Furthermore, Bajgain et al. [88] identified that hypertension, DM, and heart diseases were the most frequently reported comorbidities in patients with COVID-19 across global hotspot areas.

Medications commonly used in the management of hypertension or DM, such as insulin, do not aggravate the overall health condition in COVID-19 patients with co-morbidities [87]. Thus, people with underlying medical conditions, must adopt appropriate COVID-19 preventive measures, continue to take the medications prescribed for their comorbidities under supervision as instructed, and restock them as necessary to avoid shortages [86,89]. Due to the high susceptibility and severity of COVID-19 among people with comorbidities, such patients should be prioritized globally for receiving the vaccination [87].

The provisions for essential medications (such as insulin), routine screening, and monitoring services, were disrupted during the pandemic [90]. Presently, the goal is to control the transmission and effects of COVID-19 globally. Nonetheless, healthcare providers should meet the basic health needs of all categories of people worldwide. For patients with DM, basic health services include the routine messaging of patients and susceptible groups regarding the probable risk of COVID-19 infection measures for the prevention and management of their condition; provision for continuous support and assistance through telephone calls, video calls, and house visits while adhering to COVID-19 preventive measures; and ensuring rapid and uninterrupted accessibility to all essential medications [90]. Additionally, routine screening and monitoring should continue during the pandemic for cancers and other medical conditions to prevent late and missed diagnoses of diseases [91].

Vaccination programs for other preventable diseases should be continued globally. Primary health providers should critically evaluate the immunization status of all categories of people, with particular attention to children and patients with comorbidities [92]. Those falling behind in their immunization schedules should be contacted so as to prevent severe public health crises that might ensue from vaccine-preventable disease epidemics, especially since schools are gradually reopening for in-person learning [92]. Yang et al. [93] and Conlon et al. [94] determined that complete vaccination against other preventable diseases, such as influenza, might confer some level of protection against COVID-19, thereby reducing severe outcomes, infection rates and the burden on the healthcare system.

There has been an exponential rise in stress, anxiety, depression, sleep problems, fear, and mental distress globally during the COVID-19 pandemic, particularly in female medical personnel, patients with COVID-19, and family members of patients suffering from COVID-19 [95,96]. Guerrini et al. [97] identified some of the major contributors to abnormal psychological manifestations in the population, including lower earnings, younger age, Latino or Hispanic ethnicity, employment or work exposure to COVID-19, living with first-responders and healthcare personnel, COVID-19 status, reduced participation in healthy behaviors (such as physical exercises), and increased participation in unhealthy behaviours.

Over the years, the associations between depression, stress, and human health have been established through alterations in the normal functioning of the immune system and the production of various inflammatory properties that promote numerous infections and metabolic diseases in individual [98]. Short-term stress may initiate a possible sympathoadrenergic-mediated increase in chemotaxis and adhesion molecule expression, which distributes immune cells to locations of infection and/or inflammation. However, long-term stress disrupts this process, rendering this mechanism unbeneficial [99]. Furthermore, long-term stress alters and disrupts the ability of the immune system to secrete or produce antibodies in response to a vaccine, thus making a chronically stressed individual susceptible to infections [99].

Depression and anxiety exert severe effects, including death, in patients with COVID-19 during the pandemic. Wang et al. [100] determined that participants with diagnosed mental health problems had a significantly higher risk of developing COVID-19 with severe outcomes than those without mental health problems, as evidenced by the higher rates of COVID-19-related mortality, particularly in those with dementia. Furthermore, late-life anxiety heightened the risk of developing COVID-19, while late-life depression was associated with a higher risk of infection and severe outcomes [100]. Thus, interventions to improve and promote the mental health of highly susceptible people during the pandemic should be adopted globally [100,101].

Meditation and mindfulness can be employed by medical personnel, patients, caregivers, and the global population to improve mental health conditions during the pandemic [102]. Mindfulness-centered stress reduction methods effectively improve depression, anxiety, pain status, and stress in populations of all ages [102], at a reduced financial cost while complementing the treatment of COVID-19 [102,103].

Physical exercise reportedly prevents anxiety and depression, regardless of the age and gender of the individual [104]. Specifically, higher levels of physical exercise prevent post-traumatic conditions and agoraphobia [104]. During the COVID-19-induced confinement, the long periods of inactivity predisposes the public to numerous health risks and mental health problems, such as anxiety, stress, and depression; participating in physical exercise is effective and should be used by healthcare workers and the general public to improve their mental health [105,106]. Furthermore, yoga techniques improve sleep quality, prevent anxiety and stress, and strengthen the immune system responses to infections [107]. In the case of lockdowns, restrictions, and work demands, the internet is a good alternative for the dissemination of yoga techniques [107].

A balanced diet, prebiotics, and probiotics offer some benefits for the improvement and promotion of positive mental health [108]. The routine consumption of diets that are abundant in edible fibers and omega-3-polyunsaturated fatty acids (≥5 g/d) may confer protection against stress, depression, and anxiety [108,109].

Social support is important for health and can be seriously affected by measures, such as physical distancing and confinement, for the prevention of SARS-CoV-2 transmission [110]. During the COVID-19 crisis, social support was important to ensure connectedness [110]. Constant communication with distant family members and friends during the pandemic prevents stress and anxiety, dissatisfaction with life, and instances of loneliness and sadness [111]. Computer-mediated communication, which involves human communication via the use of two or more electronic devices, provides individuals with opportunities to remain in contact with loved ones and family members, while not compromising COVID-19 prevention measures [112]. Thus, the provisions for fast and high-quality internet, quality communication gadgets, and improved media literacy of the general population are warranted globally [112].

The most viable and effective nontherapeutic method to halt and abate the spread of SARS-CoV-2 is the adherence to all basic COVID-19 preventive measures [113]. As the pandemic lingers on and more contagious variants of the SARS-CoV-2 emerge, such as the omicron and delta variants, the general public must continue to follow all COVID-19 preventive measures: physical distancing of at least 1 m, selfquarantine, isolating when infected with COVID-19 or after contact with an infected person and immediately contacting the health service provider, regular and appropriate use of face covers, routine hand washing with soap and clean running water for a duration of at least 20 seconds, respiratory hygiene, frequent surface cleaning, and avoidance of crowded places [114]. If strictly followed and adequately implemented, these measures may also help prevent the spread of influenza and numerous other upper respiratory infections [115]. Even after an individual has been fully vaccinated for COVID-19 or has recovered from COVID-19, all preventive measures should be followed to prevent reinfection and subsequent spread [116].

Numerous COVID-19 vaccines have been proven safe and effective by numerous regulatory bodies globally [117]. Complete immunization against COVID-19 effectively protects and prevents against contraction and spread of the SARS-CoV-2 [118]. Furthermore, it prevents the development of severe illness and outcomes if infected with COVID-19, which is particularly important in people with underlying medical conditions [118]. Children should be prioritized for COVID-19 vaccination. Presently, health authorities have approved the use of the Pfizer, Johnson & Johnson, and Moderna vaccines for children over the age of 5 years, and individuals over the age of 18 years in the United States [119]. Currently, there is no COVID-19 vaccine which has been approved for children under the age of 5 years. To receive the COVID-19 vaccine, individuals should contact their personal healthcare providers or seek information at the nearest vaccination centers. The public should regularly contact their healthcare providers in case of changes to vaccination requirements, such as the need for booster shots, especially in the face of the emergence of new contagious variants of SARS-CoV-2.