Protective immunity is not a guarantee
The antibody titers are peaking at ≈1 month after symptom onset. The SARS patients were tested positive for antibodies to SARS for up to 3 years. Was the level of antibody sufficient? What does that mean for a current pandemic?
UK’s National COVID Testing Scientific Advisory Panel, published preliminary data to discuss whether individuals recovered from COVID19 have detectable antibodies in the blood. The presence of antibodies and the titers signal protection against possible reinfection.
Samples of confirmed COVID19 patients were tested for IgM and IgG levels. According to this paper, an infected person with confirmed positive PCR test for COVID-19 infection, experience rises in IgM and IgG levels during acute infection. After the initial recovery, the high levels of IgG persist for some time, and the patient is presumably immune for reinfection during that period.
Best case scenario: antibody-positivity and antibody titers confer protective immunity. And the level of antibodies is sufficient and lasting.
The IgG levels to COVID-19 were detected in all tested individuals (31) after ten days. IgG titers rose during the first three weeks and began to fall by eight weeks, but remained above the detection threshold. Sever symptomatic infections did not result in a higher level of antibodies.
These findings are in line with the immune response to other coronaviruses, and it tends to fade quickly so the same virus can reinfect initially recovered person a year or two later.
However, the lesson of SARS tells a different story. Center for Disease Control and Prevention from China, reported and published the data gathered after the SARS outbreak in 2003–2004. According to the paper that appeared on www.cdc.gov back in 2007, based on the available data from 176 patients, “the “SARS-specific antibodies were maintained for an average of 2 years, and significant reduction of immunoglobulin G–positive percentage and titers occurred in the third year. Thus, SARS patients might be susceptible to reinfection >3 years after initial exposure”. The actual levels of IgG were not measured. The testing in China was focused on the presence of specific antibodies to SARS but not on the titers.
The important finding was consistent with recent COVID-19 data “the general trend of IgM titers peaking is ≈1 month after symptom onset”.
What is unknown:
- the durability of protective immunity for COVID-19
- the specificity of protective immunity ( 33 mutated versions of COVID-19 has already been recognized)
- the feasibility, utility, and safety of a potential vaccine
- similarities and differences between SARS and COVID-19 viruses
- the level of antibodies that warrants no reinfection