Covid-19 has now infected over 13 million people around the world. But can they contract the virus again?

It seems that every day there is a new story about a person who is diagnosed with Covid-19 infection for a second time. Last week a physician from New Jersey claimed that two of his patients contracted the virus again, just two months after recovering from their initial infection. Similar stories have circulated around the country, prompting people to question whether they are truly safe from re-infection after an initial bout with the disease.

So far, experts say these anecdotes don’t amount to definitive proof.

“It’s certainly not cause for alarm,” said Dr. William Schaffner, a professor of preventative medicine and infectious diseases at Vanderbilt University Medical Center. “We have anecdotes where the scientific basis is partial, but it’s not really tied up in a nice red bow. It’s not complete.”

Now experts are scrambling to understand just how long people are protected from infection after they’ve already recovered from Covid-19.

Infection with Covid-19, or any virus for that matter, prompts the body to activate one's immune systems to attack the active virus directly and also create antibodies, some of which may help protect us against future infections.

Scientists don’t yet know for sure if it’s possible for people to be infected a second time, but two recent studies – one from China and one from the United Kingdom – found that the antibodies that fight against future infection faded within a few months.

The first study, from Wanzhou, China, and published in Nature Medicine, found that neutralizing antibodies faded quickly – after just eight weeks – in both asymptomatic and symptomatic people. And a study published by King’s College in London, which has not yet been peer-reviewed, found that neutralizing antibody response may begin to decline just three to four weeks after Covid-19 symptoms initially emerge. The study also found a more durable antibody response in patients with more severe symptoms.

Although these recent studies have hinted that our antibodies may diminish faster than many had hoped, experts say antibodies are not the whole story, and it’s possible that other parts of the immune system might still offer some level of protection.

“The duration of immunity is unclear, but it does make sense that we may start seeing cases of re-infection with the novel coronavirus, as with other common coronavirus infections," said Dr. Todd Ellerin, director of infectious diseases at South Shore Hospital. In theory, given a long enough time after recovery, re-infection is “plausible,” he said.

The novel coronavirus, or SARS-CoV-2, belongs to a large family of coronaviruses that are known to cause illnesses varying from the common cold to severe infections such as severe acute respiratory syndrome, or SARS.

Based on research from other coronaviruses, experts say there is a wide range in how long immunity lasts after an initial infection. For several of the common cold causing coronaviruses, prior infection might protect you for about six to 12 months. Other types of coronavirus – such as the Middle Eastern Respiratory Syndrome, or MERS virus – may, however, result in several years of immunity.

Given this variation, there is ongoing research to determine if the virus that causes Covid-19 will behave more like MERS or more like the common cold.

Experts caution that immunity from a viral infection is a complex, multifold process. Although the exact length of immunity after Covid-19 infection is not yet determined, it is certainly possible re-infection may occur.

That means even people who have recovered should exercise caution, practice social distancing, wear masks and wash their hands, according to the Centers for Disease Control and Prevention.

“Having it once does not give you free reign to go out and act as if there is no coronavirus around. I think we still need to take precautions because there is still a lot we don't know," said Simone Wildes, an infectious disease physician at South Shore Health.

Although recovery after a confirmed coronavirus infection “certainly provides immunity over a short term,” people “should not change [their] infection prevention behaviors,” said Ellerin. “Re-infection at some point in the future remains a possibility.”

Experts agree that we won’t know whether reinfection is possible until the virus has been with us for years, rather than months. But with over 13 million confirmed cases worldwide and limited anecdotes of possible re-infection, so far large-scale series of cases of re-infection have not occurred – a somewhat reassuring sign.

“We don’t know whether Covid will follow the same pattern" as other coronaviruses, said Schaffner, but “it wouldn’t surprise a lot of people” if it did.

• Shantum Misra, M.D., is a senior resident in internal medicine at Dartmouth-Hitchcock Medical Center and a contributor to the ABC News Medical Unit.