In the midst of COVID-19 epidemic, some people may have the following question: I have had viral influenza or even pneumonia before. Does that mean I am immune to the "virus"? Does it mean that I will not be infected by COVID-19?

Some COVID-19 survivors may want to know whether they can still get reinfected after recovery?

This problem can be actually divided into three smaller ones.

First, what is the standard of recovery?

Generally, the patients can be released from quarantine and discharged if they meet all of the three standards below or if they have been transferred to other departments to treat other diseases. That is, the patients have recovered from COVID-19^[1]:

• The body temperature has been restored to normal for over 3 days;
• Significant improvement of respiratory symptoms (dry cough, nasal obstruction, and running nose);
• Negative for two consecutive nucleic acid testings of respiratory pathogens (the time interval between two samplings is at least one day).

Second, does the body have immunity after rehabilitation?

Yes.

Similar to severe acute respiratory syndrome (SARS), COVID-19 is caused by a novel type of coronavirus, that is, 2019-nCoV^[2]. For the COVID-19 patients who have already recovered and be discharged from hospital, 2019-nCoV has been basically eliminated from their body and antibodies against 2019-nCoV have been produced. In other words, they now have the immunity.

Lastly, let's answer the ultimate question: Will the patients get reinfected so long as they have recovered and acquired the immunity?

Possibly!

During the press conference held by the National Health Commissions on January 31, the relevant responsible person mentioned that "according to the general rules of viral infection, certain antibodies will be produced after viral infection, thus giving protection to humans"^[3].

However, the COVID-19 patients may be still reinfected under the following two situations:

• The patients are in poor health and the concentration of antibodies produced against COVID-19 is relatively low. Although the antibodies are still able to recognize the "old enemies" and put up the "fight", their "fighting capacity" is too low and the antibodies may still "lose the battle", resulting in reinfection.
• The virus has mutated. That is, the "old enemies" have camouflaged so that the antibodies cannot recognize them. 2019-nCoV is an RNA virus featured by structural instability. It is likely to mutate during the spread. As a result, the previously produced antibodies may be ineffective against the mutated virus, leading to reinfection^[4].

Therefore, the recovered patients still have the risk of getting reinfected and therefore should strengthen protection[5].

Besides, another question may be raised: "Can those who have already been infected by other viruses still be infected by 2019-nCoV?

Yes!

The human body can indeed remember the past "invasions" and produce certain antibodies to prevent itself from being invaded again.

However, the antibodies have strong memory but poor "divergent thinking ability". They can only recognize the "enemies that have once invaded", but not the new enemies. In other words, they are unable to recognize 2019-nCoV that has never been there before.

Hence, for those who have recovered from the common flu or other types of viral pneumonia, the previously produced antibodies provide no immunity against 2019-nCoV. These people still need to strengthen protection to avoid being infected.