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Omicron's ability to evade immune response against previous infections and vaccination has highlighted the ability of coronavirus to adapt in ways that allow it to maintain its replication in the population. An analysis on reinfections recorded in Qatar was recently published as a letter to the editor in the New England Journal of Medicine and assesses whether prior infection protects against potential reinfections.
Professors of the School of Medicine at the National and Kapodistrian University of Athens, Gikas Majorkinis and Thanos Dimopoulos (Dean of EKPA) say the study, like other similar studies, highlighted Omicron's superior ability to evade reinfection. More specifically, for the Alpha, Beta and Delta strains, prior infection protected against symptomatic reinfection at levels higher than 85%, while with the Omicron mutation this level drops to 56%. However, in the same analysis it appears to offer significant protection against severe disease and Covid-19 although the number of severely ill in this study was very small.
The findings of the studies on the rate of reinfection are important for calculating the likelihood of achieving herd immunity naturally. Herd immunity is the phenomenon where the transmission of a pathogen to the population is prevented by those who are immune (either naturally or through vaccination) to such a level that the epidemic is stopped. The theory predicts that this occurs when the number of transmissions of each vector to the general population i.e. the active breeding number, known as Rt, falls below unity. For this to happen a proportion of the population must have sterile immunity, i.e. not catch the virus, or if they do catch it, not transmit it. This percentage is calculated crudely through the baseline replication number, known as R0, which is the number of transmissions each vector makes to a population that has no immunity at all. The baseline replication number in the original SARS-CoV-2 strains was 2.5-3.0 and corresponds to a need for a sterile immunity rate of 60 to 67%. The estimate for the Alpha, Beta and Delta strains is that the baseline reproduction number increased reaching about 5 for the Delta variant resulting in a sterile immunity rate required for herd immunity of up to 80%. As becomes apparent such a rate of sterile immunity based on the data published in the study (which shows that re-infection with the Delta variant reaches 90%) could only be achieved when the disease in the population reached 88% of the population. So the reason we started to see a slowing of the epidemic in Greece in early December was that the combination of natural disease and vaccination was approaching 88%, leading to a de-escalation of the epidemic, say the two professors.
With the advent of Omicron, however, even assuming no increase in baseline breeding numbers, the maximum sterile immunity rate that can be achieved is 56% so there could be no herd immunity even if 100% of the population had been infected with previous strains. However, we should note that these reinfection rates refer to infection with different strains and not strains of the same variant (e.g. first Delta and then Omicron). Therefore we do not know the rate of Omicron to Omicron reinfection which would reasonably be higher than 56%. In any case, the reinfection data show that the targets for SARS-CoV-2 vaccination programmes have been revised upwards with rates exceeding 90% for all adults and reaching 100% for age groups over 60.
Source: protothema.gr
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