Back in January, in the early stages of Israel's highly successful vaccination rollout, all the experts agreed that it would be impossible to achieve herd immunity against the highly infectious British variant of Covid-19, without being able to inoculate under-16s.
Meanwhile, Israel has vaccinated 'only' 5.35 million people out of a population of 9.3 million, including almost 5 million who have had two Pfizer jabs, and the number of new infections keeps falling. Despite the full opening of the economy, the Passover and Independence Day holidays, and the return to school, the R rate remains anchored well below 1. Even so, Israel Covid-19 project head at the Ministry of Health Prof. Nachman Ash insists that Israel has not achieved herd immunity. So what have we achieved?
Prof. Ran Balicer, the Chief Innovation Officer at Clalit Health Services and Director of the Clalit Research Institute sounds very optimistic in answering this question. "We were on the verge of the abyss and got ourselves out of it by the skin of our teeth. If the vaccination rollout had been delayed, even by a few weeks, we would have seen completely different results."
But it is still not herd immunity he emphasizes. "Herd immunity means that the virus would fade out even if we behaved as if it was still 2019. Every country has its own natural behavior. Herd immunity sees a fall in new cases even if people are behaving naturally. But what is natural behavior in Israel is not natural behavior in Sweden."
Israel has reopened its economy but is still not behaving as if it was 2019. People must still wear masks indoors according to government instructions not all extra-curricular activities for children have restarted and people prefer to sit outside at restaurants and cafes, even though regulations do not require it. People are still cautious about physical contact, especially if they know that somebody has not been vaccinated. It's possible that the R rate will again rise above 1 and then we will realize that we have not achieved herd immunity.
Prof. Balicer said, "In order to know when Israel has reached herd immunity, you must first identify the parameter called R0, which is how many people are being infected when there are no vaccinations and when people are behaving naturally. We estimate that the British variant has an R0 of 4, in other words, in an unvaccinated population behaving naturally without any restrictions, every infected person will infect an average of four others." In Israel the R0 is higher because it is densely populated and there is a culture of less social distancing.
He explains that with an R0 of 4, at least three out of every four peole must be vaccinated in order to reach R=1. In other words three out of every four people who would otherwise have been infected, won't in practice be infected. A vaccination level of 75% of the overall population, including children is therefore the level that we are aiming for. But it's more complicated because the vaccination is not perfect and on the assumption that it protects 80% of people from viral transmission, we need to have over 90% of people vaccinated to have full and complete natural herd immunity. And we are far from there. 80% of the adult population is vaccinated but nearly one third of the population are under 16.
Prof. Balicer said, "When we get to 90% or more we can say to the public, hug, kiss and go wild. Even if you come to work sick, it won't make any difference to coronavirus."
That was the situation in 2019 regarding measles. Behavior was natural and nobody took precautions against measles, but nevertheless, vaccinations were successful in preventing it. But that is not the situation at the moment with Covid-19.
So what we see at the moment is partial herd immunity or according to Balicer an indirect protective effect.The few restrictions still in place, official and unofficial, together with partial herd immunity, are enough to make the virus fade, so that even the unvaccinated aren't encountering infected people, and are not being infected.
But Balicer warns, "If we really open up everything, the situation could still change. I don't believe that we would get that easily to an R of 1.2 or 1.3, as we had in January, unless we are talking about a new variant. It doesn't seem possible at the current level of the vaccinations, and I don't see us returning to a situation that endangers the health system's surge capacity. But there could be a situation in which the virus spreads between children and unvaccinated people, with vaccinated people being infected from time to time.
"Even in such a situation, the behavior of the population would also change, even without government regulated restrictions, and we could restore control. When the numbers are very low, it's easy to stop the spread, and that buys us time to respond and so we are relatively calm."
Published by Globes, Israel business news - en.globes.co.il - on April 19, 2021
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