A version of a guide for EndNote isn't useful for CSL-based styles.

Are you submitting to the Bulletin of the World Health Organization or to some other WHO serial? To a WHO book or report?

The WHO BULLETIN and other serials follow the NLM or a Vancouver citation style. Zotero has several versions of these styles and (from personal experience as a reviewer assigned to check reference accuracy for WHO) there is a need for consistency with any particular numeric style rather than any rigid punctuation demand. (If you are collaborating with other authors, all should use the same scheme.) edit: I've seen reference numbers superscripted or in round or square brackets -- just try not to mix the punctuation. (WHO will edit and manually typeset your manuscript. They will be really pleased to receive the Zotero NLM style with PMIDs/PMCIDs. Be sure to include DOIs for all items that have them. Your reference list will be thoroughly checked for both accuracy and pertinence/relevance. Don't cite something where the results do not support what you asserted and don't make a typo in your reference list such that the reference cannot be easily found. Don't cite statements in an introduction/background and don't reference cited statements of others from the discussion. These are important for any serious journal but especially so for the WHO Bulletin. A single hollow reference will equal doom. )

The Bulletin provides a link to the:
International Committee of Medical Journal Editors (ICMJE) recommendations for the conduct, reporting, editing and publication of scholarly work in medical journals (http://www.icmje.org/icmje-recommendations.pdf) See page 18 for reference and bibliography guidelines (and specification of NLM). The WHO is primarily interested in assuring that authors follow the extensive ethical standards in the link above. New (2024) guidance is provided that 1) restricts or limits citations to articles published in journals from predatory publishers; and 2) gives further guidance concerning the number of allowed preprint service citations. [Both of these are related to peer review issues.]

I haven't been able to find an online version of the WHO style for books and reports dated 2024 only earlier ones:

https://www.unaids.org/sites/default/files/sg13_web_v4 pdf - adobe reader.pdf
see p 36

https://iris.who.int/bitstream/handle/10665/36842/WHO_PUB_TPS_93.1.pdf?sequence=1&isAllowed=y

Edit: even for books and reports the numeric (NLM) style is preferred. Only under specific circumstances is a "Harvard (author date)" style allowed.

I recommend using the NLM with PMIDs/PMCIDs style. It is likely that you will need to add the style to those offered in your Zotero program. Doing so is really easy. When you are asked to select a citation style, at the bottom of the list of installed styles you will see a link to Manage Styles. Click that and a small window will open. Click the Get Additional Styles link. Enter NLM in the Style Search field. You will see a list of options. Click the link to the style that is labeled "National Library of Medicine (grant proposals with PMCID/PMID)". When asked if you want to add the style to Zotero click yes or OK.

I admit that I haven't checked this style to see if it uses et al (but I'm almost sure that the style includes all authors). WHO doesn't want et al in favor of a full author listing. Thus, maybe use one of the other NLM styles or modify the style I recommended to remove the et al policy.

WHO bibliographic style is any NLM style but from experience with the WHO Bulletin, the editors like to have options to access full text on PubMed Central if it is available. If you use any NLM or Vancouver style your manuscript is not likely to be rejected on the basis of your bibliographic style alone. When WHO refers to the 2024 style guide they are more interested in the ethical concerns than in the details of a bibliographic style.