Three mpox infections nobody ever diagnosed, and no idea how

During the 2022 mpox outbreak, transmission was **primarily through close
skin-to-skin contact, including sexual contact**, and **people experiencing
homelessness were disproportionately affected by severe mpox.** Two things
followed from that combination, and neither was good: **prevalence and
transmission pathways in that population were not known**, and **people
experiencing homelessness were not specifically recommended for mpox
vaccination during the outbreak.**

San Francisco's health department, which had been finding cases among people
experiencing homelessness through July–October 2022, invited a CDC field team
to look.

How the survey was done

25 October – 3 November 2022, at 16 sites — seven shelters, five
service centres, two supportive housing locations and two encampments — chosen
because they had served at least one person with confirmed mpox or served
people at increased risk.

284 people were recruited. Eleven (4%) were excluded for living in their own
private residence. Of the remaining **273, 240 (88%) consented to blood
collection and blood was successfully obtained from 209 (77%).**

Two details say a lot about doing research with this population:

• When venipuncture failed or was unlikely to succeed — needle aversion,
  veins that could not be seen or were damaged — **a microneedle device was
  offered** that passively collects capillary blood from the upper arm
• Surveys ran in English or Spanish, or through a phone interpreter for
  any other language, and each participant received **up to two $25 grocery
  gift cards**

What the blood showed

Serum was tested for antiorthopoxvirus IgG; IgM followed for anyone with
detectable IgG or reporting higher-risk behaviours.

The vaccination numbers are their own quiet finding — barely half of those who
said they had been vaccinated against mpox had detectable IgG.

And then **three participants who reported no vaccination and no previous mpox
infection** had detectable antibodies: possible undetected infections.

The part that does not fit the outbreak

**None of the three reported sexual contact during the preceding month, being
transgender, being a gay or bisexual man, or having a rash at the time of the
survey.**

Some reported **sharing utensils and smoking devices, and spending time around
or touching someone with a rash.** But **the timing of exposure is not known
and could have preceded the survey period**, so:

**The transmission route for the three possible undetected mpox infections
could not be determined.**

The report does not resolve this, and says so. Mpox can spread through
contaminated objects and close non-sexual contact, though **that risk is
considered lower.**

Why an infection goes undetected

Subclinical, atypical or mild disease — or **barriers to seeking or
accessing health care**, which the authors note could apply to the people in
this survey. Both mechanisms end in the same place: an infection that
transmitted, was never counted, and never appeared in the outbreak's picture
of how it was spreading.

Which is the argument the report makes: **tailored outreach so that prevention
messaging and vaccination reach people experiencing homelessness** — a group
that was not specifically recommended for the vaccine while the outbreak was
running.

Source: Centers for Disease Control and Prevention, MMWR.