Israel has not achieved herd immunity despite the stunning success of its coronavirus vaccine campaign.
“Herd immunity is protection that is either from vaccination or previous exposure to COVID, that results in a situation where the virus has no power to circulate in our lives [once] we have returned to our normal lives before the pandemic,” explained Dr. Dan Yamin, head of the Laboratory for Epidemic Modeling and Analysis in Tel Aviv University’s Faculty of Engineering.
Normal lives means no face masks or social distancing; no capsules or limits on gatherings.
There are three reasons for the lack of herd immunity.
The first is variants.
The British variant is at least 45% more contagious than the original Wuhan strain, meaning that the threshold for achieving herd immunity, which was at first believed to be around 60%, has increased to somewhere near 80% in Israel where 99.5% of new cases are people who are infected with the mutation.
Yamin and his colleagues published a research paper – “The rise of SARS-CoV-2 variant B.1.1.7 in Israel intensifies the role of surveillance and vaccination in elderly” – on the infectiousness of the British variant earlier this year, based specifically on Israeli data in which the 45% increase was identified. Other studies have shown a range in increased infectiousness of between 30% and 70%.
Moreover, Yamin explained, while the Pfizer vaccine was found to be effective against the British variant, it could prove less effective against new variants – meaning that Israel will not be able to declare herd immunity until it knows the virus can no longer circulate.
The second reason is transmission.
While studies have shown that the vaccine is highly effective at reducing or even blocking symptomatic infection, there is still a question about asymptomatic infection.
“We know from other vaccines, such as the vaccine against pertussis or rotavirus, that vaccination does a very good job of preventing symptomatic infection, but does a less good job at preventing asymptomatic cases,” Yamin said.
Those people who may have contracted coronavirus before vaccination and developed symptomatic cases are now much more likely to be asymptomatic if they contract the virus at all. But it is still unclear if they can transmit the virus to someone else.
The latest studies in Israel are starting to reveal that people who were vaccinated have a reduced viral load – meaning that they are likely less infectious than someone who was not vaccinated and caught the disease – and are therefore less likely to spread corona.
“The studies are starting to tell us that the vaccine does a good job in preventing transmission,” Yamin said. “However, we don’t know to what extent.”
Finally, the third reason is children.
To date, around 5.1 million Israelis have been vaccinated with at least one dose of the Pfizer vaccine. But there is still a large, unvaccinated population in Israel.
Around 30% of the Israeli population is children under the age of 16 who are not eligible for vaccination. In some areas of the population, such as in the haredi (ultra-Orthodox) community, the percentage is much higher – 50%.
Additionally, while more than 90% of people over the age of 50 have gotten the jab, there are still 250,000 more who have not been vaccinated. And about 800,000 eligible vaccinees between the ages of 16 and 50 have not been inoculated either.
“I would put a lot of effort into getting those people between the ages of 16 and 50 vaccinated,” said Prof. Daniel Cohen, a member of Israel’s Epidemic Treatment Team and the former head of Tel Aviv University’s School of Public Health. “But things will be much better when there is the possibility to start vaccinating kids under 16.”
Pfizer has completed enrollment for a clinical trial of children between the ages of 12 and 16 and the data are expected to be released over the summer.
As noted, experts believe that around 80% of the population would have to be vaccinated or recovered to achieve herd immunity, but this is just an estimate.
In the case of measles, around 90% to 95% of the population need to be vaccinated to have indirect protection, Cohen said. In contrast, in the case of other vaccine preventable diseases, such as diphtheria and polio, only about 80% of the population needs to be vaccinated.
It’s possible that in certain areas of the population, herd immunity has already started to appear, he said – such as in areas with less youth or greater vaccination rates – but that is different from achieving national herd immunity.
Yamin said that in the near future, the coronavirus should enter a state of “silent epidemic” meaning that there would not be massive infection and certainly less strain on the health system. This is especially so because the majority of children do not develop serious cases of COVID-19.
But he said that while the disease is still spreading, Israel should proactively test the population to catch asymptomatic cases before they spread, especially in areas where people could be at the highest risk, such as in senior living facilities.
“We should now put all of our efforts to protect those at high risk,” Yamin said.
Cohen added that while Israel is enjoying an exciting downward trend in morbidity – only around 2.4% of people screened on Saturday tested positive – the population should still act with caution and continue to combine vaccination with other preventative steps, such as masks and social distancing where applicable.
“The data are very encouraging,” Cohen said, “but we still cannot speak about a significant herd immunity at this stage.”