2- Thoughts on Covid-19 and IgG-IgM Antibody Testing.pdf

Thoughts on Covid-19 and IgG/IgMAntibodyTesting

Immunological Response to Viral Infection

SARS-CoV-2 is the virus responsible for Covid19. From an immunological standpoint, viral infectionsfollow the same pattern,with different timeframes andmagnitude of responses.

First the virus starts replicating before symptoms even appear, this is the incubation period.

Thenthe virus replication generates antigens that trigger immune response and symptoms appear. Thisis the start of the acute phase.

Seroconversionfollows.The body then produces IgM antibodies as a short-term response to fight the infection. This is when viral loadand antigen levels should start to decrease.

Finally, IgG antibodies are produced. These are long-term antibodies, specific tothe virus. The body will continue producing them beyond the recovery period, for months andeven years, to prevent reinfection.

In the case of Covid-19 (SARS-Cov-2) the durationof the differentstagesofinfection isnot completely ascertained. This virus seemsto be replicating in different body areas at different time, like: Upper Respiratory Track,Lower respiratory track, Bloodand Anal-Rectum (e.g.

Zhang W, etal.). And an important percentage of patientsexhibit mild or no symptoms.

Molecular tests, like PCR,which detect the virus nucleic acids themselves, work best in the early infection cycle, when the virusis actively replicating, and viral load is detectable.

Antibody testing, like IgG/IgM, works well once the body has had the time to initiate an immune response.

Antigentesting works best in the early stage ofsymptomsand would be a good complement to Antibody testing, allowing to close the time-window to infection.

IgG/IgM Test Relevance

In theory, antibody testing is not as relevant for patients who exhibit veryrecent symptoms. However, inpractice, it seemsmost Covid-19 patients exhibit mild symptoms or are asymptomatic altogether. These patients are therefore not likely to be testedfor molecular tests whichwould typically be carriedout concurrentlywith acute symptoms.

However, this patient group could be contributing largely to the disseminationof the virus among the community. (China guidelines, Joint WHO-Commission report).

Antibody testing could establishwhether patients have beenexposed to the virus, regardless of symptoms.

1

In the case of Covid-19 (SARS-Cov-2) Jia Xingwang etal. () reporteda 72.73 % ofpositive IgM/IgGon suspected patients withmildorno symptoms and Negative PCR. Presumably, these patients viral load had decreased to the point that PCR was not helpful anymore, but antibodies showed exposure.

Furthermore, Zheng W.et al. reported that IgM and IgG can be detected earlier than with other viruses.Zheng W. et al. reported that, at 5 Days after the onset ofsymptoms, IgM and IgG were present in100 %of the patients tested.

This shortertimeframe with SARS-Cov-2 (Covid19) makes antibody testing more relevant.

Lastly Jia Xingwang et al. ()reported 82% positive IgM/IgG on thosewithClinical symptoms and PCR positive. The authors concludedthe clinical relevance of IgG/IgM antibody testing in symptomatic and asymptomatic patients.

Dr Sunetra Gupta from Oxford University (UK) developed an epidemiological model.It suggested that SARS-CoV-2 spread in the community evenbefore the first severe caseswere identified. They recommended immediately large-scale antibody testing.

Antibodytesting outcome

Positive antibody testing would confirm virus exposure, whether symptoms are or have been present:

IgMantibodieswould indicate that theinfection happened recently,and thatthe body may still be actively fighting the virus.

IgG antibodieswouldindicate that the infection is abit older andthat the body has controlledthe acute phaseof the infectionof the virus.

This information, and further testing if necessary, taken togetherwith the patient’s clinical presentation, symptoms history, and their direct contacts with exposed or sickpeople, would enable decisions from healthcare providers and authorities:treatment, observation, isolation…

Large-scale antibody testing would also ascertain, at apopulationlevel, the spread of the SARS-Cov-2 virus, and where to focus efforts for containment and control.It could also potentially help assess people’s ability to end isolation, return to work or their families.

This would be particularly important when assessingthe status of healthcare and essential servicesworkers.

2