A study conducted in Israel compared reinfections and breakthrough infection in people who had recovered from COVID-19, those who hadn't had the virus but received the vaccine and a group that had experienced both of these events.
There has been a lot of speculation about whether those who have previously contracted COVID-19 developed an immunity to the virus. It was assumed that this was the case as this was the initial hypothesis involving herd immunity. Frequently, a certain percentage of the population has to be able to get an illness for it to spread. It the number is lower than this percentage the disease will spread at an increasing rate while if it's higher then the number of people contracting the illness will decrease.
Herd immunity happens in the latter case, when a large enough proportion of the population becomes immune to the disease which will eventually eliminate its abiltiy to spread. This protest the entire population not just those who are immune to the disease. Herd immunity can happen when enough individuals have recovered from an illness and developed antibodies against future infection if they are again exposed
There are obvious limitations to relying on herd immunity from natural infection alone. These include the fact that a certain number of people will die from the virus and the fact that at least 70% of the population in the U.S. would need to have recovered from the virus and developed antibodies in order to achieve herd immunity in the first place.
The other issue is that for herd immunity to occur, people must actually develop antibodies that prevent them from developing the disease again. When the pandemic first began, it was assumed that you couldn't get the virus twice. However, this was quickly disproven as individuals began experiencing reinfections. While the rates of the disease in those who had previously contracted and recovered from it were lower, than in those who'd never had it before, it was still possible for those with the antibodies to become reinfected.
Herd immunity can also be developed by vaccinating enough of the population to eliminate transmission. But while the vaccine effectively decreases the liklihood of severe illness, hospitalization and death, it also doesn't provide complete immunity. Additionally, there are the problems of vaccine hesitancy and refusal, and widely circulating variants that may be less responsive to the vaccine.
It's come to be accepted that herd immunity is unlikely because of these factors. Studies have turned to determining whether there are things we can learn from those who have recovered the virus that will help improve our ability to prevent it's transmission. A recent large scale study in Israel examined differences in reinfection and breakthrough infection rates for those who had recovered from the virus but hadn't been vaccinated, those who had received the Pfizer two dose vaccine but hadn't had the virus and those who had experienced both of these events.
Subjects were 778,658 individuals. Of those, 62,883 were unvaccinated and had recovered from COVID-19, 673,676 were fully vaccinated with the Pfizer vaccine and hadn't been exposed to the virus, and 42,099 were vaccinate and had recovered from the virus prior to vaccination.
It was found that participants who were vaccinated but had not been exposed to COVID-19 had a significantly increased risk of becoming infected with the Delta variant compared to unvaccinated individuals who had recovered from the virus. Vaccinated individuals also had a much higher risk for symptomatic COVID-19 and for requiring hospitalization due to breakthrough infections.
Participants who had been recovered from the virus and were vaccinated with with one dose of vaccine were found to have greater protection against the Delta variant than those in the other two groups.
(In March, 2021 the Israeli Ministry of Health revised its guidelines and allowed individuals over the age of 16 who had recovered from the virus to receive one dose of the Pfizer vaccine, after at least a 3-month-interval had passed since the date of infection. Prior to June 2021, Israel would not allow anyone under the age of 16 to receive a vaccination.)
This study demonstrated that the resistance that comes from having the virus provides longer lasting and more robust protection against infection, developing symptomatic illness and required hospitalization caused by the Delta variant, compared to the resistance that results from the Pfizer two-dose vaccine. Individuals who were previously infected with COVID-19 as well as given a single dose of the vaccine gained additional protection against the Delta variant above what was experienced by those with only immunity conferred from having the virus.
The investigators note that these findings may not necessarily generalize to individuals who had recieved a different vaccine or to variants other than the Delta variant.
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