**1.3 Incubation period**

The incubation period for COVID-19 is generally within 14 days following exposure, with most cases occurring approximately four to five days after exposure. However, determinations of the incubation period can be imprecise and may differ by the method of assessing exposure and the specific calculations used for the estimate.

#### **1.4 Initial presentation**

Among patients with symptomatic COVID-19, cough, myalgias, and headache are the most commonly reported symptoms. Other features, including diarrhea, sore throat, and smell or taste abnormalities. Pneumonia is the most frequent serious manifestation of infection, characterized primarily by fever, cough, dyspnea, and bilateral infiltrates on chest imaging. Although some clinical features (in particular smell or taste disorders) are more common with COVID-19 than with other viral respiratory infections, there are no specific symptoms or signs that can reliably distinguish COVID-19. However, development of dyspnea approximately one week after the onset of initial symptoms may be suggestive of COVID-19.

The range of associated symptoms includes; Cough in 50 percent, Fever in 43 percent, Myalgia in 36 percent, Headache in 34 percent, Dyspnea in 29 percent, Sore throat in 20 percent, Diarrhea in 19 percent, Nausea/vomiting in 12 percent, Loss of smell or taste, abdominal pain, and rhinorrhea in fewer than 10 percent each.

In a meta-analysis of observational studies, the pooled prevalence estimates for smell or taste abnormalities were 52 and 44 percent, respectively (although rates ranged from 5 to 98 percent across studies) [7]. However, the rate of objective smell or taste anomalies may be lower than the self-reported rates.

Most subjective smell and taste disorders associated with COVID-19 do not appear to be permanent; in a follow-up survey of the 202 patients with COVID-19, 89 percent of those who noted smell or taste alterations reported resolution or improvement by four weeks [8].

Although not noted in the majority of patients, gastrointestinal symptoms (eg, nausea and diarrhea) may be the presenting complaint in some patients. In a systematic review of studies reporting on gastrointestinal symptoms in patients with confirmed COVID-19, the pooled prevalence was 18 percent overall, with diarrhea, nausea/vomiting, or abdominal pain reported in 13, 10, and 9 percent, respectively [9].

Nonspecific signs and symptoms, such as falls, general health decline, and delirium, have been described in older adults, particularly those over 80 years old and those with underlying neurocognitive impairments.

Dermatologic findings in patients with COVID-19 are not well characterized. There have been reports of maculopapular, urticarial, and vesicular eruptions and transient livedo reticularis. Reddish-purple nodules on the distal digits similar in appearance to pernio have also been described, mainly in children and young adults with documented or suspected COVID-19.

#### **1.5 Acute course and complications**

Symptomatic infection can range from mild to critical. Some patients with initially non-severe symptoms may progress over the course of a week. In one study of 138 patients hospitalized in Wuhan for pneumonia due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), dyspnea developed after a median of five days since the onset of symptoms, and hospital admission occurred after a median of seven days of symptoms [10].

#### *1.5.1 Recovery and long-term sequelae*

The time to recovery from COVID-19 is highly variable and depends on age and pre-existing comorbidities in addition to illness severity. Individuals with mild infection are expected to recover relatively quickly (eg, within two weeks) whereas many individuals with severe disease have a longer time to recovery (eg, two to three months). The most common persistent symptoms include fatigue, dyspnea,

*Clinical Characteristics of COVID-19 Infection DOI: http://dx.doi.org/10.5772/intechopen.99088*

chest pain, cough, and cognitive deficits. Data also suggest the potential for ongoing respiratory impairment and cardiac sequelae. Some patients who have recovered from COVID-19 have persistently or recurrently positive nucleic acid amplification tests (NAATs) for SARS-CoV-2. Although recurrent infection or reinfection cannot be definitively ruled out in these settings, evidence suggests that these are unlikely.
