How do the calculation and the goal ofherd immunity with the presence of the Delta variant, so much more contagious?
Does the Delta variant infect more people?
The Delta variant would be more contagious than the 40-60% compared to the Alpha variant, still dominant in Italy and which, in turn, was about 50% more contagious than the virus that characterized the first wave of Covid. The Delta’s contagiousness increase figure should be “on paper” closer to 60%, but it decreases when the virus moves on land where, unlike in 2020, there is a substantial percentage of vaccinated or cured, which are however largely protected also from reinfections. This obviously concerns the EU, the US and the UK, from which the most data on the Delta variant also come. On 6 June the journalist of the Times Tom Calver compiled an explanatory graph (see image below) of Delta’s reproduction number R0 compared to Alpha and the ancestral virus: in 2020 without lockdown and social distancing measures, a person could infect about 3 others (R0 equal to 3) , with Alpha a person could infect 4 and with Delta it would be 6, but they could also be up to 10 (according to the latest data).
The reproductive capacity of SARS-CoV-2 and of the two variants (source Twitter Tom Calver)
Breeding number and herd immunity
R0 (reads r with zero) represents the so-called “basic reproduction number”, ie the average number of secondary infections caused by each individual infected in a population that has never been in contact with a particular pathogen. Rt instead describes the rate of contagiousness after the application of the measures able to contain the spread of the disease.
These numbers they change the “threshold number” to be reached (as a percentage of the population vaccinated or cured) to have herd immunity and stop the spread of the virus. The so-called “herd immunity” is a mechanism whereby, when the majority of a population is immune to an infection (because they have contracted it or been vaccinated), the pathogen does not find subjects to infect indirectly protected even the few who are still susceptible.
The Delta variant in Italy: things to know
The assumptions on the percentages to be achieved
The calculations are difficult to do and the figures dance a little: during the first wave, when vaccines did not yet exist, the aim was about 60-70% of the population vaccinated (or cured) to achieve herd immunity. With the Alfa variant the threshold had reached 70-80% e now the calculation must be revised upwards again. Roberto Battiston, internationally renowned physicist and full professor at the University of Trento, custodian of Agenas data on the pandemic in the Italian regions, in an interview with Courier service assumes that we must arrive 88% of the population vaccinated. “We should reach 80% of full-cycle vaccinated people,” virologist Francesco Menichetti, head of infectious diseases at the Pisa hospital told Adnkronos. It therefore seems unlikely “achieve herd immunity in late September», As Andrea Costa, Undersecretary of Health, told Sky TG24. Roberto Burioni wrote on Twitter: “If, as it seems, the Delta variant has an R0 between 8 and 10, politics must be taken seriously and QUICKLY the vaccination obligation for everyone or you risk big. The virus is no longer what we have known, it has become much more dangerous ”, referring to Delta’s ability to spread and infect people who are still susceptible because they are exposed or not vaccinated.
The highway vaccines
The only data in favor is the high number of vaccinated people in Italy and also of those recovered, which are officially over 4 million, but in reality they are more, since not all the positives have been recorded (especially in 2020). This implies that the threshold for immunity could theoretically drop, albeit not by tens of percentage points. It happened in Israel: when 55% of the population was vaccinated, all restrictions could be lifted. Now that the Delta has arrived there, it is estimated that without vaccinating the children herd immunity will no longer be achieved.
July 4th 2021 (change July 4th 2021 | 07:39 am)
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