CDC Covid-19 guidance: how long are you immune after catching coronavirus?

Early on during the Covid-19 pandemic there was a widespread belief that once a patient had contracted the novel coronavirus and developed antibodies, it would be impossible for them to be infected again with SARS-CoV-2. 

WHO: no evidence of protection from second infection 

However, several instances in South Korea suggesting that patients there who had recovered from the disease had later returned positive results and the case of at least 14 sailors aboard the USS Theodore Roosevelt recovering from Covid-19 then testing positive again has left scientists to question whether having had the virus and developing antibodies is a guarantee that it cannot be contracted multiple times. 

As the World Health Organization stated in April when discussing immunity passports: “There is currently no evidence that people who have recovered from Covid-19 and have antibodies are protected from a second infection.”

To that end, the United States Centers for Disease Control and Prevention (CDC) was obliged to clarify in a statement issued at the weekend that its updated isolation guidance does not imply that a person is immune to re-infection with the novel coronavirus.

"Contrary to media reporting today, this science does not imply a person is immune to reinfection with SARS-CoV-2, the virus that causes Covid-19, in the 3 months following infection", the CDC said in statement.

The agency said the latest data simply suggests that re-testing a person in the three months after initial infection is not necessary unless that person is showing symptoms of Covid-19 and the symptoms cannot be associated with another illness.

On 3 August, the CDC had updated its isolation guidance based on the latest science about Covid-19 showing that people can continue to test positive for up to three months after diagnosis and not be infectious to others.

"People with Covid-19 should be isolated for at least 10 days after symptom onset and until 24 hours after their fever subsides without the use of fever-reducing medications", the CDC said.

“There have been more than 15 international and U.S.-based studies recently published looking at length of infection, duration of viral shed, asymptomatic spread and risk of spread among various patient groups. Researchers have found that the amount of live virus in the nose and throat drops significantly soon after Covid-19 symptoms develop. Additionally, the duration of infectiousness in most people with Covid-19 is no longer than 10 days after symptoms begin and no longer than 20 days in people with severe illness or those who are severely immunocompromised,” the statement concluded.

CDC explains "false positives" 

In July, the CDC explained a possible cause for people returning more than one positive test for Covid-19 as being false positives caused by residual but probably uninfectious traces of the virus: "Detection of viral RNA does not necessarily mean that infectious virus is present.”

In other words, especially in the US were the majority of diagnostic tests look for snippets of the virus' RNA or genetic code, the test may be picking up a piece of the viral RNA that's been left behind, rather than a fully intact, infectious virus particle.