What does it mean to say a population has “low levels of general immunity”?

Question

The UN Humanitarian Coordinator in Yemen, Lisa Grande, is quoted in several places (e.g. this CBS news article as saying (emphasis mine):

[SARS-CoV-2] is now in Yemen and may quickly spread. […] The factors are all here: Low levels of general immunity, high levels of acute vulnerability, and a fragile, overwhelmed health system.

Another phrasing (possibly a paraphrase of the same statement) is given in this short Guardian article:

The WHO has said it fears Covid-19 could rip through Yemen as the population has some of the lowest levels of immunity to disease compared with other countries.

These statements are a little ambiguous — they obvious reading is as meaning “immunity to diseases in general” but they could conceivably also be meant as “immunity to a specific disease, in the population in general”. But as I understand, no populations are believed to have specific pre-existing immunity to SARS-CoV-2/Covid-19 — or at least, the idea that existing immunity to other coronaviruses may confer some immunity to SACS-CoV-2 is quite tentative and not yet widely accepted as a significant effect. So the quote presumably is meant in the sense of “immunity to diseases in general”.

But what is this “general immunity”? All the explanations of immunity I’ve seem treat the concept of immunity as essentially disease-specific. Does it mean high rates of conditions like AIDS that reduce immune system function? Or is there some other sense in which people/populations can have higher or lower “general immunity”?

Answer 1

The term "general immunity" is not used all that often nowadays, but it seems to stand in opposition to "local immunity" which are more specialized immune responses in some regions of the body but not others. A 1923 (!) paper on this contrast mentions "Metchnikoff's phagocytic theory of immunity"... and in more recent publications Metchnikoff is called "the father of innate immunity"; this latter paper doesn't use the term "general immunity" at all. So possibly, the terms "general immunity" and "innate immunity" are used interchangeably in some quarters.

Now, why would [innate] immunity be lower in Yemen and comparably disadvantaged populations? In general, from (at least) animal studies the innate immune system was shown to be modulated by starvation, and so presumably by poor nutrition too. And the innate immune system has a few general mechanisms that e.g. in response to type I interferon trigger some specific mechanisms that slow or inhibit viral replication: Dicer, PARP 13 etc. Now viruses themselves are in an arms race with the innate immune system and some viruses have acquired ways of disrupting it.

A 2016 review on human immunity issues resulting from malnutrition notes that:

Characterization of immunodeficiency was limited by a lack of longitudinal studies, particularly for mild and moderate malnutrition. The precise nature of immunodeficiency in undernutrition therefore remains uncertain; however, the consensus from the available evidence is that both innate and adaptive immunity are impaired by malnutrition. Defects in innate immune function include impaired epithelial barrier function of the skin and gut, reduced granulocyte microbicidal activity, fewer circulating dendritic cells, and reduced complement proteins, but preserved leukocyte numbers and acute phase response.

(The same review notes that interestingly overnutrition, e.g. resulting in obesity also seems to impair some immune functions. On that [aside] angle, there has been a bit of controversy whether (higher) BMI is risk factor for Covid-19, if all other associated problems like diabetes etc. are factored out.)

Answer 2

I think they're just pointing out that Yemen has a very poorly functioning public health system with generalised poor nutrition so that the individuals of the population have little reserve to fight infection.

More than 80% of Yemen's population lacks food, fuel, drinking water and access to health care services, which makes it particularly vulnerable to diseases that can generally be cured or eradicated elsewhere in the world. The health care system has been decimated by years of unrelenting war in Yemen.

They also suffered terribly from cholera which is a treatable disease.

Despite being a completely treatable disease, thousands of people have died from the outbreak of cholera unleashed in Yemen in 2017. 

https://www.icrc.org/en/where-we-work/middle-east/yemen/health-crisis-yemen