Our Sources
The primary authorities behind the health information on the Virus Questions network.
The sourcing tiers
"We use primary sources" is what every health site says. What matters is what happens when sources disagree, when only weak evidence exists, or when the figure a reader wants has never been published at all. The network resolves that with an explicit three-tier scheme, applied per figure rather than per page.
| Tier | What qualifies | Used for |
|---|---|---|
| Tier 1 | CDC, WHO, and NIH directly. | The default for any clinical or epidemiological claim. |
| Tier 1.5 | Other official government or supranational statistics bodies — ECDC, the US Census Bureau, the UN Population Division, national health ministries. | Denominators, national case counts, and regional data the Tier 1 agencies do not publish. |
| Tier 2 | Peer-reviewed literature, cited by DOI, with systematic reviews and meta-analyses preferred over single studies. | Mechanistic detail and clinical figures no agency publishes. Every Tier 2 figure additionally carries a confidence rating. |
Aggregators, media, advocacy organisations and wikis have no tier. That is deliberate: there is no representation for them in the data model at all, so a figure from one of those sources cannot be entered even by mistake.
Two rules that do the real work
A derived figure inherits its weakest input's tier. Some figures are arithmetic rather than numbers lifted from an agency page — a case fatality rate computed as deaths divided by cases, for instance. Where that happens, both inputs are recorded separately with their own source, geography, and reporting period, and the result is rated no higher than the weaker of the two. A Tier 1 numerator over a Tier 2 denominator produces a Tier 2 figure, never a Tier 1 one. The inputs are published alongside the result, so the arithmetic can be checked rather than taken on trust — see any page in the Virus Risk Perspective for worked examples.
A figure that does not exist is marked as not existing. Where no source publishes a comparable figure, the value is recorded as unavailable and rendered as such. It is never estimated, interpolated, carried over from a different population, or replaced with a single-study result standing in for a general rate. This is enforced in the data model rather than left to discipline: the schema rejects a silent zero, and roughly half the figures in the comparison dataset are unavailable for exactly this reason.
The consequence is worth stating plainly, because it cuts against how comparison tables usually read: a blank cell is not a low number. It means the measurement does not exist. Treating it as zero would make the least-studied diseases look like the safest ones.
Centers for Disease Control and Prevention (CDC)
The CDC is the primary U.S. public health authority. We use CDC guidance for clinical descriptions of disease, transmission routes, treatment and prevention recommendations, outbreak surveillance, and travel advisories. The CDC is the first source we consult for any U.S.-relevant health claim.
World Health Organization (WHO)
The WHO is the international public health authority. We use WHO guidance for global outbreak status, Public Health Emergency of International Concern (PHEIC) declarations, global epidemiological data, and international travel guidance. For any outbreak with international scope, WHO situation reports are a primary reference.
U.S. Food and Drug Administration (FDA)
The FDA is the U.S. authority for vaccine approvals, treatment authorizations (including Emergency Use Authorizations), and drug safety. We use FDA sources for any claim about approved or authorized vaccines and treatments.
National Institutes of Health (NIH) / NIAID
NIH and its National Institute of Allergy and Infectious Diseases (NIAID) publish foundational research and clinical guidance on infectious diseases. We use NIH sources for research summaries, mechanism-of-disease explanations, and clinical trial data.
European Centre for Disease Prevention and Control (ECDC)
The ECDC provides European epidemiological surveillance and guidance. We use ECDC sources where European-specific data, outbreak tracking, or guidance is relevant — particularly for hantavirus (where European strains differ significantly from U.S. strains) and for internationally circulating diseases.
Peer-reviewed journals
For mechanistic explanations, clinical data, and emerging research not yet reflected in authority guidance, we cite peer-reviewed literature. Primary journals used:
- The New England Journal of Medicine (NEJM)
- The Lancet
- Nature Microbiology
- Morbidity and Mortality Weekly Report (MMWR, CDC)
- PubMed-indexed studies in infectious disease, virology, and epidemiology
All cited studies are linked directly to PubMed or the journal's stable URL. We note when a study has been retracted or significantly disputed.
National health ministries
For outbreak-specific data — case counts, geographic extent, outbreak declarations — from affected countries, we cite the relevant national health ministry (e.g., DRC Ministry of Health for the 2026 Ebola outbreak). These are used as primary data sources, not as authoritative clinical guidance.
What we do not use as sources
- News articles (we link to news from CoronavirusQuestions.com and EbolaQuestions.com, but do not source health claims from news)
- Other health websites, including well-known aggregators like WebMD or Healthline
- Preprints not yet peer-reviewed (cited only as "preliminary findings" when directly relevant)
- Social media posts or expert opinions not published in peer-reviewed form