The American Psychological Association has an entire article dedicated on their website to discuss the implications of social isolation. There are physical, cognitive and emotional implications of having limited or no social interactions. On the current times when quarantining has become a necessity, the implications prove to be even more prevalent.
Nearly half of the 20,000 U.S adults reported feeling alone, either some time or always, according to a meta analysis by Julianne Holt-Lunstad and colleagues (2015), a professor of psychology and neuroscience at Brigham Young University. The lack of social connections and loneliness increased the chance of health risks such as alcohol use disorder, and smoking. Her study found social isolation and loneliness to have twice as harmful health implications as that of obesity.
A study led by Newcastle University epidemiologist Nicole Valtorta and colleagues (2016) linked a 30 percent increase in the risk of a stroke or coronary heart disease to loneliness.
Other implications of loneliness are higher degrees of stress, gross negative impact on sleep, resulting in harm to the body. It can also culminate to cause anxiety and depression.
Researchers at Florida State University College of Medicine also found that it was associated with a 40% increase in the risk to develop Dementia. Participants in this study rates their loneliness and social islolation levels, as well as completed a battery to asses their cognitive abilities every two years.
Social isolation is a very widely researched area in the field of psychology. There have been studies to understand the cause of feeling lonely, and thereby come up with strategies to help the individual cope with the feeling of loneliness. The underlying effort is to identify when one starts feeling disconnected, and differentiation between being alone and feeling lonely. There are also efforts to minimize and address the negative thoughts that arise while one goes through social isolation. The addition of social skills and refined social support are good tools for a person who goes through feeling lonely.
References:
https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1000316
https://heart.bmj.com/content/102/13/1009
https://academic.oup.com/psychsocgerontology/article/75/7/1414/5133324?login=true