Last week, the US Centers for Disease Control and Prevention updated its quarantine guidelines online to say that people who have recovered from Covid-19 do not need to quarantine or get tested again for up to three months — but the agency clarified in a statement to CNN on Friday that does not mean that people are immune to reinfection.
“People who have tested positive for COVID-19 do not need to quarantine or get tested again for up to 3 months as long as they do not develop symptoms again,” the CDC guidance stated.
A CDC spokesperson said the guidance is “based on the latest science about COVID-19 showing that people can continue to test positive for up to 3 months after diagnosis and not be infectious to others.”
Yet “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 latest data simply suggests that retesting someone in the 3 months following initial infection is not necessary unless that person is exhibiting the symptoms of COVID-19 and the symptoms cannot be associated with another illness,” the CDC statement said.
“I think this is an incredibly sticky wicket, as the British would say,” said Dr. William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University School of Medicine in Nashville, who was not involved in the CDC guidance.
“We think antibodies correlate with protection, but we don't really know that yet,” Schaffner said.
Practical applications, Schaffner added, are still a long way off. Can a person who has antibodies begin to date or stop wearing a mask, for example? Absolutely not, he said emphatically.
Our adaptive immune system, designed to target a specific and previously recognized invader, contains two main cellular components — B cells and T cells. Studies on immunity so far have focused mainly on B cells, a type of white blood cell programmed to find and bind to foreign invaders.
Once it has attached itself to a virus such as the novel coronavirus that causes Covid-19, a B cell “copies itself and churns out antibodies, eventually creating a mega-army of neutralizers for that particular invader,” said CNN Chief Medical Correspondent Dr. Sanjay Gupta in an explainer on virus immunity.
“Unfortunately, a few recent studies have found that antibodies to this particular coronavirus can fade away pretty quickly especially in people who have had mild cases of Covid-19,” Gupta said.
A pre-print paper (meaning it has not been published in a peer-reviewed journal) released in July found antibody responses appear to decline 20 to 30 days after Covid-19 symptoms first appear.
“This study has important implications when considering protection against re-infection with SARS-CoV-2 and the durability of vaccine protection,” the researchers wrote in the paper.
In June, a small study found that people who have coronavirus infections but never develop symptoms could have weaker immune responses to the virus.
That study found that a group of about three dozen Covid-19 patients who were asymptomatic had levels of antibodies that were significantly lower than what was found among patients who had mild symptoms — a finding that suggests the asymptomatic patients had weaker immune responses.
In early July, a Spanish government study found that just 5% of people in Spain have coronavirus antibodies and in a potentially worrying development, the study also indicated that people's immunity to coronavirus wanes after just a few weeks. The findings show that 95% of Spain's population remains susceptible to the virus.
This means that any perceived immunity “can be incomplete, transitory and then disappear,” said Dr. Raquel Yotti, head of the Carlos III Health Institute, in a news conference at the time. The institute is a key government agency leading the study.
Hoping for a T-cell response
Science is hoping that Covid-19 will also trigger an immune response from another arm of the adaptive immune system — T cells. One type, called a memory T cell, helps the body remember a specific invader in case it comes back.
A recent study found such memory T cells in people infected with SARS-CoV-2, the novel coronavirus that creates the disease called Covid-19. That finding leads to speculation that some people might get milder cases of Covid-19 because their T-cells are reacting to exposure to similar coronaviruses encountered in the past.
The virus SARS-CoV-2 belongs to a large family of coronaviruses, six of which were previously known to infect humans. Two are deadly: SARS-CoV-1, the virus responsible for the outbreak of Severe Acute Respiratory Syndrome (SARS), which ended in 2004; and MERS-CoV, the virus that causes Middle East Respiratory Syndrome (MERS).
The other four cause the common cold, and because they circulate widely each year, it's possible many people may have already been exposed to one of these so-called “cousins” of the novel coronavirus.
“They've been studied and we know that after you had a cold, you develop antibodies to that particular human strain of coronavirus and you will be protected, but it turns out the protection begins to wane after about a year,” Schaffner said.
“Now when Covid-19 came along, we wondered how much protection are you going to get from this virus? And if you get protection, how long does it last?” he added. “We don't know that behavior yet.”
Stay the course
Another page on the CDC site, updated in June, refers to the uncertainty that exists surrounding the role of antibodies in future infections.
“Having antibodies to the virus that causes COVID-19 might provide protection from getting infected with the virus again. If it does, we do not know how much protection the antibodies might provide or how long this protection might last,” the CDC says in regard to testing.
Until science knows more about how long immunity lasts, Schaffner advised staying the course of what is known to work: hand-washing, social distancing, avoiding crowds and wearing masks.
“Don't go around looking for reasons not to wear the mask, just wear it. Get used to it,” Schaffner said. “We want to all be in this together to affirm our solidarity.”
Editor's note: This article has been updated to include comment from CDC clarifying its quarantine guidance.
CNN's Andrea Kane contributed to this report.