Is it possible to get coronavirus a second time? Another issue that is curious, please stay away from crowded environments mask, always follow the rule of 2 meters to the hygiene mask.
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always stay away from risky people 
Covid-19 disease due to the new type of coronavirus (SARS-CoV-2) was first seen in December 2019 and has affected the whole world since then. From that date until March 2021, approximately 120 million Covid-19 cases were reported worldwide, and more than 90 million people managed to recover from this disease. So, do people who survive this disease have the risk of being infected with the new type of coronavirus (SARS-CoV-2) and getting Covid-19 disease?

According to the US Center for Disease Control and Prevention (US-CDC) on September 10, 2020, until that day there were no confirmed reports that a person was infected again with SARS-CoV-2 within 3 months of being infected.
Due to the lack of human data, Rhesus monkeys were re-exposed to the same virus strain 28 days after initial infection and no recurrent infection condition occurred.

However, on September 21, 2020, the European CDC published a report addressing the recapture of SARS-CoV-2.

Getting coronavirus disease for the second time
Recurrence of infections is observed in most respiratory viruses, including human coronaviruses. Reinfection with respiratory viruses;

Weak or weakening of the initial immune response,
Reinfection with another genotype / species,
It may be due to viral load change of viruses.
Certain antibodies against coronavirus were detected in people who survived up to 24 and 34 months, respectively, after SARS-CoV-1 and MERS-CoV (Middle East Respiratory Syndrome Coronavirus) outbreaks. The negligible risk of second exposure to these viruses did not permit a clinical definition of immunity to reinfection.

The table below provides summary information about the different respiratory viruses.

epidemiological comparison of respiratory tract infections influenza covid-19 sars mers
For coronaviruses that cause epidemics, immunity has been shown to be a temporary state that lasts from a few months to a few years, and re-infection has been reported after natural and experimental infection. It is thought that the immune response after natural viral infection is incomplete and reinfection is likely.

Covid-19 vaccine platforms currently tested, such as RNA and viral vector vaccines, are designed to elicit antibody and T cell responses. However, subunit vaccines are more limited to antibody responses by eliminating the effect of active effector T cells and memory T cells.

What do the repeated Covid-19 and repeat positive PCR tests mean within 90 days of the onset of symptoms?
In some publications, the SARS-CoV-2 PCR test has been described again following negative tests and clinical recovery, or cases with symptoms consistent with Covid-19. These cases were identified over a period of up to 90 days after acute illness and may reflect persistent / fluctuating viral transmission with persistent or recurrent (recurrent) clinical disease rather than a true recurrent infection state. Other viral infections or bacterial / fungal (fungal) superinfections may also explain a co-occurring symptom attack in the presence of SARS-CoV-2 RNA residue in some of these entities. In addition, the weak again positive PCR results (cycling threshold (Ct)> 35) probably reflect the non-replicated virus, not the true active infection.

In addition, in big data from China, the median hospital stay due to Covid-19 was reported as 22 days. The median viral transmission for hospitalized cases was reported to be as long as 20 to 37 days. In other reports, viral transmission from upper respiratory tract samples was found to occur for up to 12 weeks. According to the CDC, a positive PCR over a 90-day period probably represents a long-term transmission rather than re-infection. However, the replicating (self-replicating) virus can rarely be detected for up to 10 to 20 days, and even in severe cases of Covid-19 it has often been detected later. However, for patients with suppressed immune status, this period has been reported to be even longer. In addition to long-term contamination, it has been identified that 14-17% of the patients who were discharged had PCR tests positive again.

Covid-19 infection status again
It was reported that a patient in Hong Kong was asymptomatic (no symptom) positive for the SARS-CoV-2 PCR test 142 days after the first symptomatic (symptomatic) Covid-19 attack. This case was defined as a recurrent infection of SARS-CoV-2.
Other possible recurring cases have also been reported in the USA. In this case, the disease emerged more severe as a result of the patients being exposed to the disease again at home about 2 months after the first Covid-19 attacks. Genome sequencing detected several potential variations (changes) between viruses in separate attacks. The cases were explained as being reinfected.

Both humoral and cell-mediated immunity are required to provide protective immunity against re-infection with SARS-CoV-2. It is not known whether most infected individuals produce adequate and sustained immune memory responses, such as neutralizing antibodies and memory cells.

The different severity scenarios in Covid-19 disease are illustrated below.

new coronavirus sars cov 2 covid 19 disease stages when and how is transmitted antibody
Long-term studies evaluating the persistence of SARS-CoV and MERS-CoV specific IgG antibody responses for more than 1 year after infection found that the presence of antibodies decreased over time. However, the presence of antibodies is not necessarily equivalent to protective immunity, especially if the neutralizing activity of the antibodies is unknown or known to be low or absent. Specifically for SARS-CoV-2, some previous studies have reported rapid reduction of antibody responses that were attenuated after 90 days. However, a recent longer follow-up study showed that antibody presence remained stable for 4 months.

Mutated Covid-19
Several different strains of SARS-CoV-2 from the same ancestor and their subunits were identified with varying prevalence depending on geographic location. Thousands of distinct variants of the virus, consisting of various mutations in the spike protein, including more than 400 variants (changes), have been reported. It is possible that the immune pressure will select new variants that are able to escape the immune response to previous infection.

NOTE: Variant are mutations that do not alter the general identity of the virus. Strains are mutations that introduce the virus to another identity. So if this happens, everything will go back to the beginning. Not every nucleotide change in the virus means its differentiation.

It is not certain that reinfection could be more severe or milder. A more severe event may occur as a result of antibody-induced augmentation or may depend on the severity of the variant / strain and the material used in vaccination. Observing more severe cases may also reflect selection bias against testing for symptomatic cases. The milder course of reinfection was explained by the initiation of the adaptive immune response during the first attack.

The European CDC proposes a flowchart for assessing recurrent infection in previously confirmed COVID-19 cases; however, the evaluation includes tests that are not available such as sequencing the whole genome and phylogenetic analysis (see table below for a summary).

Variable

Confirmed re-infection status

Clinical re-infection status (a)

Epidemiological re-infection

Clinical symptoms

Characteristic clinical signs

Characteristic clinical signs

Asymptomatic / symptomatic

PCR

(one must be done)

Positive

Positive

Positive

Viral culture (one must be done)

Positive

Positive

Positive

Time frame after first infection

> 90 days (b)





Isolation measures

Suggested

Suggested

Suggested

Additional findings

Viral RNA sequencing from both attacks reveals different strains

Epidemiological risk factor (known exposure or epidemic state), no other cause

Epidemiological risk factor (known exposure or epidemic state), no other cause

(a) Clinical characteristic signs of 2019 coronavirus disease (Covid-19).

(b) If improvement is proven by current known Covid-19 exposure and two consecutive negative PCR tests, it may be considered if it is less than 90 days.
Confirmed diagnosis of re-infection with SARS-CoV-2 requires:
A true first attack verification - definition of the viral load of the first attack is required. It is shown that the original sample is retested whenever possible. (Ct values> 35 may indicate possible contamination rather than true infection)
Evidence of reinfection with two SARS-CoV-2 RT-PCR test positive with Ct> 35 at different times. (Or evidence of virus replication by cell culture or detection of sub-genomic RNA)
Confirmation of infection with two different phylogenetic strains by high-throughput sequencing consistent with local epidemiology. (Evidence of two different virus variants with any set of variations between two attacks)
Note: At least one and ideally two negative RT-PCR tests should be documented on two different samples collected between the first and second episodes.

Related topic: What are the long-term effects of Covid-19 infection? 50 signs
Recurring Covid-19 condition
Recurrent Covid-19 status (defined in the literature as "recrudescence", "recurrence" or "reactivation") - Clinical recurrence of symptoms compatible with Covid-19 - accompanied by positive / persistent RT-PCR within 90 days of initial infection and epidemiological exposure is supported by the absence of the condition or another cause of the disease.
Viral culture may be positive.
Displaying the same strain by whole genome sequencing can precisely distinguish this entity from reinfection. Therefore, more research is needed to decide whether these cases could be considered for isolation.
If the SARS-CoV-2 RT-PCR test is positive again
When the SARS-CoV-2 RT-PCR is positive again, it is the RT-PCR that is positive after negative tests in an asymptomatic patient within 90 days after the first attack.
These cases are likely not representative of replicative viruses and do not require isolation.
Low viral load is often a feature of being positive again.
Result
The next few months will likely reveal the extent of re-infection situations. Fortunately to date, reinfection seems to be a rare occurrence. More research is needed to provide more accurate definitions in terms of time period, genetic modification definitions, and serological status.