there is a high risk that people have some immunity

[GoodTaste]

I can't clearly understand why "that that people have some immunity" is "a high risk"? Is it a high risk that would lead to false results?

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On the other hand, Feigl-Ding says, measuring vaccine effectiveness in Brazil may produce artificially low results because there is a high risk that people have some immunity because of previous contact with the virus. “This is a natural property of doing a trial in a highly previously infected population,” he says. This could also explain why the CoronaVac clinical trial in Brazil showed lower efficacy numbers (50%) than in other countries such as Turkey (83.5%) and Indonesia (65%).

Source: Science Chinese COVID-19 vaccine maintains protection in variant-plagued BrazilBy Sofia MoutinhoApr. 9, 2021

 

[emsr2d2]

It's difficult to show the efficacy of the vaccine there because the vaccine testing really needs to be carried out on people who have not have Covid. The prevalence of Covid in Brazil means that a high proportion of the population has probably been infected, symptomatically or not. That makes it very difficult to find enough people who are and have always been Covid-free to test the vaccine on.

 

[Phaedrus]

I can't clearly understand why "that that people have some immunity" is "a high risk"? [. . .]

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. . . measuring vaccine effectiveness in Brazil may produce artificially low results because there is a high risk that people have some immunity because of previous contact with the virus.

You chose not to embolden the "because of"-phrase, but it, too, is part of the "that"-clause complementing the noun phrase "high risk."

The meaning is that there is a high risk in Brazil that people's having previous contact with the virus will itself explain their immunity to the virus.