Bengaluru: People who have recovered from Covid-19 may have antibodies only for two to three months, said a study published Thursday in Nature Medicine.
However, the conclusions might not necessarily mean that a recovered person can get reinfected, stated the authors.
The limited findings offer clues to one of the biggest questions about Covid recovery, vaccination, and serological surveys.
The study is the first to determine the existence of antibodies in asymptomatic patients and characterise them. Previous studies have shown that infected patients who display symptoms do develop antibodies as well, but it remained unclear how long these lasted.
Another paper published in Nature Medicine, also released Thursday, found that even small amounts of antibodies could work to protect against the virus.
Asymptomatic vs symptomatic
The new study compared 37 asymptomatic patients with 37 symptomatic patients who had all been quarantined after testing positive in the Wanzhou People’s Hospital in China.
The authors found that the immune response in asymptomatic people was weaker as compared to those who showed symptoms. They also found that antibody levels started falling much faster in asymptomatic patients than in symptomatic ones.
Antibodies reduced in 40 per cent of the asymptomatic group as compared to 12.9 per cent in the symptomatic group during the early convalescent or recovery stage — about eight weeks after individuals were discharged from the hospital.
IgM, the antibodies produced during the preliminary stages of the infection, were found to last longer in symptomatic patients. When tested for IgM antibodies three to four weeks after falling sick, it was found in 78.4 per cent of the symptomatic people and 62.2 per cent of the asymptomatic group.
Body response during the infection
During the acute phase of the coronavirus infection, which is defined as when nasal swabs test positive for the virus, asymptomatic patients displayed lower levels of IgG antibodies, those produced in the later stages of the infection that help carry an individual to recovery.
However, during the early convalescent phase, the IgG count dropped in 93.3 per cent of asymptomatic individuals, lower that the 96.8 per cent of the symptomatic group.
Neutralising antibodies, which deactivate the spike protein the coronavirus uses to gain entry into cells, decreased in 81.1 per cent in the asymptomatic group and in 62.2 per cent in the symptomatic during recovery.
Asymptomatic people also displayed lower levels of pro- and anti- inflammatory cytokines, which are proteins responsible for transmitting signals between immune system cells. Cytokine levels found in asymptomatic individuals were comparable to completely healthy individuals.
Antibodies to other coronaviruses such as SARS and MERS are thought to last at least a year. Some SARS antibodies in individuals have lasted for two years, while some MERS antibodies have been detected 34 months after recovering from the infection.
The study also found that asymptomatic people were shedding the virus, i.e., releasing a replicated virus to the environment, for far longer than symptomatic people did.
Symptomatic people shed the virus for 14 days while asymptomatic people did so for 19. However, it is unclear if the virus being shed at later stages is capable of infecting others or is just viral RNA remnants.
Viral shedding data also varies across studies; previous studies have shown viral shedding occurring for up to 24 days after the onset of symptoms.
“Measurable virus shedding does not equate with viral infectivity, and further evaluation is needed,” wrote the authors of the paper.
The findings have implications for vaccine and serological surveys. The authors recommend that RT-PCR and serological testing should be used in conjunction to determine accurately the extent of asymptomatic transmission.
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