Why is someone still testing positive for COVID-19 (Coronavirus Disease 2019) 17 to 19 days after initial infection?

From the Guidelines

A person may still test positive for COVID-19 17 to 19 days after initial infection because viral RNA fragments can remain in the body even after the person is no longer contagious. This is particularly common with PCR tests, which are highly sensitive and can detect even small amounts of viral genetic material. Most people are not infectious after 10 days from symptom onset (or from the first positive test if asymptomatic), despite continuing to test positive. The CDC and many health authorities no longer recommend testing to end isolation precisely because of this phenomenon. Instead, they suggest time-based strategies for ending isolation: typically 5 days of isolation followed by 5 days of masking when around others, assuming symptoms are improving and the person has been fever-free for 24 hours without fever-reducing medications. The persistence of positive tests occurs because the test detects viral fragments rather than viable, replication-competent virus. These fragments can take weeks to clear from the body completely, even though the immune system has already neutralized the active infection and the person is no longer able to spread the virus to others, as supported by studies such as 1 and 1.

Some key points to consider:

• The sensitivity of antigen tests is lower than that of nucleic acid amplification tests (NAATs), especially for asymptomatic individuals or when testing is performed late in the course of infection 1.
• The specificity of antigen tests is high, but a negative test result may require confirmation with a standard NAAT if clinical suspicion for COVID-19 remains high 1.
• The performance of antigen tests may change as new variants emerge, highlighting the need for ongoing monitoring and research to improve test accuracy 1.
• Time-based strategies for ending isolation are recommended, rather than relying on testing, to minimize the risk of unnecessary isolation and to reduce the burden on healthcare systems 1.

Overall, the persistence of positive COVID-19 tests beyond 17 to 19 days after initial infection is a common phenomenon that does not necessarily indicate ongoing infectiousness. By understanding the limitations of COVID-19 testing and following recommended guidelines for isolation and masking, individuals can minimize their risk of transmitting the virus to others and reduce the spread of COVID-19 in their communities.

From the Research

Background on COVID-19 Testing

• The COVID-19 pandemic has led to the development and use of various diagnostic tests, including rapid antigen tests and molecular-based tests, to detect SARS-CoV-2 infection 2, 3.
• These tests vary in sensitivity and specificity, with antigen tests being less sensitive but more convenient and rapid than molecular tests 2, 3.

Factors Influencing Test Results

• The sensitivity of antigen tests is higher in symptomatic individuals, especially in the first week of illness when viral loads are higher 2, 3.
• The sensitivity of antigen tests also varies between brands, with some meeting the World Health Organization's (WHO) acceptable performance criteria of ≥80% sensitivity and ≥97% specificity 2, 3.
• The timing of testing is crucial, as antigen tests may not detect infection in the early or late stages of the disease 2, 3.

Interpreting Test Results

• A positive result on an antigen test does not necessarily confirm the presence of infectious virus, as the test may detect viral antigens even after the individual is no longer infectious 4.
• Negative results on an antigen test do not rule out infection, especially in asymptomatic individuals or those with low viral loads 2, 3.
• The use of rapid antigen tests in combination with nucleic acid detection may improve the accuracy of COVID-19 screening, especially at points of entry 4.

Considerations for COVID-19 Screening

• The choice of test depends on the setting, population, and purpose of testing, with antigen tests being suitable for rapid screening and molecular tests being more accurate but requiring laboratory facilities 2, 3.
• Serial testing, such as every three days, may improve the detection of asymptomatic infections and reduce the rate of missed detection 4.
• It is essential to follow proper testing procedures and interpret results in the context of clinical and epidemiological information to ensure accurate diagnosis and effective management of COVID-19 5.