Okay.
The second part of this question is whether a person can have a flare up.
This has been circulating in the media since COVID-19 patients in Korea were reported to
test positive shortly after recovering from the disease.
The three causes could be reinfection, a relapse, or inconsistent RT-PCR tests.
An overly sensitive RT-PCR test might be detecting non-infective RNA fragments rather than infectious
viral particles.
A study from the Korean CDC trace 790 close contacts of 285 people who retested positive
and found that none of the close context became infected.
Researchers tested respiratory samples from 108 of the repositive cases and could not
isolate the intact virus.
Of the 23 repositive cases from whom the first and second serum samples were obtained, 96%
were positive for neutralizing antibodies.
It seems unlikely for the SARS-CoV-2 virus to cause a flare up in the future because
SARS-CoV-2 is a single stranded RNA virus and does not have the enzyme reverse transcriptase.
Without reverse transcriptase, the virus is not able to insert into DNA and cause flare
ups in the future, like other viruses, such as HIV, varicella zoster virus, or herpes
virus, which all have a reverse transcriptase component.
The last part of this question is can a person become reinfected?
This has to do with the adaptive immune system.
Generally after recovery from a virus, the body has developed specific antibodies, such
as memory B cells to prevent future reinfection.
How long immunity lasts depends on the virus.
With some viruses, such as measles morbillivirus, which causes measles, infection confers almost
universal, lifelong lasting protection.
On the opposite end of the spectrum, individuals infected with HIV often have antibodies that
do nothing to fight HIV.
Some viruses like HIV use multiple strategies to evade antibodies.
So what about other coronaviruses?
Antibody, titers, and individuals that survived SARS-CoV-2 or MERS-CoV infections often waned
after two to three years or were weak initially.
However, one study showed that convalescent sera from SARS patients taken three in a half
years after disease onset did neutralize the SARS-CoV-1 virus and even somewhat cross neutralized
SARS-CoV-2 entry into the cell via the ACE2 receptor.
That suggests that SARS-CoV-1 antibodies are still able to neutralize the SARS-CoV-1 virus
three and a half years after the initial antibody response.
This study also provided some preliminary evidence that antibodies made in response
to other coronaviruses may provide some cross immunity against SARS-CoV-2.
Although another study found that this was not the case, but we will come back to that
later.
In SARS-CoV-2 survivors, the CD8+ T memory cells persisted for 6 to 11 years, suggesting
there may be long-term immunity.
Additionally, coronavirus is responsible for the common cold induce antibody responses
that neutralize the coronavirus that causes the common cold, but they don't appear to
last and wane over the course of one to two years.
That means people can be reinfected with the exact same coronavirus one to two years later.
It is still unclear how long antibodies generated from the SARS-CoV-2 infection last and whether
or not they confer protection.
The immune system is not perfect.
And over many years, the memory B cells may not recognize the virus that they were programmed
to recognize.
So there's no guarantee that you can't get reinfected at some point.
Also viruses often mutate, or they find other ways to evade the immune system.
There was a strain of the SARS-CoV-2 virus.
It has a mutation in a region of the virus that antibodies bind to.
Nonhuman primates infected with SARS-CoV-2 that recover and have antibodies do not become
reinfected when re-exposed to the virus 28 days later.
We know that people infected with SARS-CoV-2 do develop antibodies against the virus.
A study found that out of 285 patients in China with COVID-19, 100% of them developed
antibodies against the SARS-CoV-2 virus within 19 days of viral exposure.
The ability of those antibodies to neutralize the virus was not determined.
But as I just mentioned, nonhuman primates that developed antibodies after recovering
were not reinfected after being exposed to the virus a second time.
And a study of 10 patients in China found a single dose of convalescent plasma from
patients that have recovered from COVID-19 and contained antibodies against SARS-CoV-2
resulted in a clearing of the virus in the patient's blood within seven days, clinical
symptoms improved significantly within three days, and lung lesions improved within seven
days.
Anecdotes from other hospitals in the U.S. report the same.
Studies of the common cold coronaviruses, SARS-CoV-2 and MERS-CoV, indicate that virus-specific
antibody responses wane over time.
And in the case of common cold coronaviruses, antibody responses may only result in partial
protection from re-infection.
These data suggest that immunity to SARS-CoV-2 may diminish following a primary infection.
And further studies will be required to determine the degree of long-term protection.
There are a few large serological surveys being led by Dr. Michael Bush, who is an MD
PhD at the school of medicine at UCSF, to look at antibody responses that are waning
and to assess herd immunity.
The handful of rare cases where people have tested positive after recovering from the
disease could also be due to testing errors as mentioned previously.