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This memo is an update of an earlier version, and the findings are based on rapid searches in PubMed, EMBASE, and supplementary searches for pre-prints. One researcher went through all search records, selected and summarised the findings. In the current situation, there is an urgent need for identifying the most important evidence quickly. Hence, we opted for this rapid approach despite an inherent risk of overlooking key evidence or making misguided judgements.
We identified 20 new original papers from the database search and by manual searching of reference lists that were relevant to our research questions. This rapid review now includes 36 studies. Half of the included studies were pre-prints that had not been through peer review, and many studies had very small sample sizes.
Does primary infection with SARS CoV-2 result in immunity, and if so, how long does the immunity last?
We found limited evidence on immunity after infection with SARS-CoV-2. One study on rhesus macaque monkeys suggests that primary infection with SARS-CoV-2 may protect against reinfection. The study was small and did not provide any information on the duration of immunity. Two studies showed sustainable IgG levels one to two years after SARS-Cove infection, but whether this finding is generalizable to SARS-CoV-2 has still to be determined, also whether sustained levels of antibodies provide full or partial protection against reinfection.
Is there cross-protection from SARS-CoV-2 infection after infection with seasonal corona viruses (sCoVs)?
There is no direct evidence for cross-protection from SARS-CoV-2 infection after infection with sCoVs.
How long does it take to develop SARS-CoV-2 specific antibodies, and what is the proportion of patients presenting seroconversion?
Seroconversion rate and timing varied across studies and between IgM and IgG antibodies. Results from some of the studies suggest a median seroconversion timing around 10-14 days after disease onset, while some studies suggest a longer time (28 to 30 days or longer) for all patients to seroconvert. We believe that much of this variation is due to differences in the test sensitivity, but may also be due to differences in the immune response between different patient groups.
Does the rate of seroconversion and/or the timing depend on the severity of SARS-CoV-2 infection?
The results for this question was mixed. While some studies reported no relationship between seroconversion and severity of COVID-19 disease, evidence from other studies suggest that a more rapid and higher antibody response may be related to the severity of disease. Also, seroconversion may not be a prerequisite for virus clearance, since asymptomatic patients, and people with undetectable levels of antibodies still manage to clear the virus.
Can antibodies be transmitted from women infected with SARS-CoV-2 to the foetus via placenta and thus confer immunity in the infant?
Results from two small studies (including in total 7 neonates) suggest that antibodies from SARS-CoV-2 infected women may be transmitted to the foetus during pregnancy, but the evidence is uncertain.