What each clinical trial phase does and doesn't prove
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A plain-language guide to reading herpes (HSV) vaccine news by trial phase. Each phase answers a different question, and early-phase results say nothing about whether a vaccine prevents disease. A trial existing or finishing is not the same as a vaccine working — the Phase 3 Herpevac trial ran to completion but missed its main goal, and as of 2026 no HSV vaccine has cleared Phase 3 to approval.
When a new herpes vaccine makes headlines, the most important detail is often the one buried deepest: which stage of testing the result came from. A clinical trial — a study that tests a vaccine or drug in living participants — is run in a sequence of numbered phases, and each phase is built to answer a different question. An early phase can look encouraging while telling you nothing about whether the vaccine actually prevents disease. Learning to read the phase correctly is the single most useful skill for interpreting vaccine news. [S7]
Before humans: preclinical studies
Every candidate begins in the laboratory, well before any person receives it. In this preclinical stage — research done in cells and in animals rather than people — scientists check whether the vaccine is safe and whether it produces the biological effect they are hoping for. For herpes simplex virus (HSV) in particular, this is where a note of caution belongs: results seen in animals frequently do not carry over to people. One 2024 review of HSV vaccine efforts found that candidates showing promise in early development, including animal studies, have often failed to reproduce those results in human trials. Preclinical success is a starting point, not human evidence. [S1]
Phase 1: the first test in people
Phase 1 is the first time a vaccine is given to humans. These studies are small — typically 20 to 80 volunteers, usually healthy — and their focus is narrow: is the vaccine safe, and does it provoke an immune response? That second property is called immunogenicity, meaning the vaccine’s ability to stir the immune system into action, for example by prompting the body to make antibodies. [S7]
Here is the catch that trips up a lot of coverage: immunogenicity is not the same as protection. Showing that a vaccine makes antibodies does not prove it prevents disease. For HSV the gap is even wider, because the specific immune response that actually predicts protection has not been established. A 2024 immunology review notes that protection appears to depend heavily on a particular kind of immune cell — tissue-resident memory CD8+ T cells, immune cells that take up long-term residence in tissue and guard against the virus returning — rather than tracking neatly with the antibody measures that are easiest to read out in an early trial. So a strong immune signal in Phase 1 is a reason to keep going, not evidence that the vaccine works. [S2]
Phase 2: refining the approach
If Phase 1 clears the safety bar, a candidate moves to Phase 2. These studies are larger — often several hundred people — and they do more work: they help pin down the right dose, gather additional safety information, and look for the first preliminary hints of effectiveness. Those hints are suggestive, not definitive. A Phase 2 result can point in a promising direction without confirming that the vaccine prevents disease. [S7]
Phase 3: the real test
Phase 3 is the large, late-stage efficacy trial — the study designed to measure efficacy, meaning whether the vaccine actually prevents the disease it targets under real-world conditions. These trials usually enroll several hundred to several thousand people and compare a vaccinated group against an unvaccinated (or comparison) group to see whether vaccination makes a measurable difference. This is the stage that is meant to settle the question. [S7]
It is also the stage that shows why a trial existing, or even finishing, is not the same as a vaccine succeeding. The Herpevac Trial for Women was a Phase 3 study of a glycoprotein-based HSV-2 vaccine candidate. (HSV-1 and HSV-2 are the two related herpes simplex viruses; HSV-1 is most often associated with oral herpes and HSV-2 with genital herpes, though either can infect either site.) The trial enrolled 8,323 women who were negative for antibodies to both viruses and ran to completion. It did not meet its main goal: the vaccine was not efficacious with regard to the primary end point — the single pre-specified outcome a trial is built to measure, here the prevention of genital herpes disease caused by HSV-1 or HSV-2. A finished, well-run Phase 3 trial produced a negative headline result. [S5]
“Completed” on a registry is not “it worked”
This distinction matters when reading trial registries such as ClinicalTrials.gov. A registry reports a study’s administrative status — whether it is recruiting, active, or completed — not its scientific results. “Completed” means the study finished enrolling and following its participants. It does not mean the vaccine was found to be effective, and it does not mean the vaccine failed either; it simply means the study ran its course. Results live in the published data, not in the status field. [S7]
What this does and does not mean
So when you read that a herpes vaccine “showed promising results,” the first question to ask is: which phase? A Phase 1 result is about safety and immune response, not prevention. A Phase 2 result offers early hints of effectiveness, not proof. Only a completed Phase 3 efficacy trial can show whether a vaccine prevents disease — and, as the Herpevac trial demonstrated, even reaching that stage does not guarantee a positive answer. It is also worth keeping the prophylactic–therapeutic distinction in view: a prophylactic vaccine aims to prevent infection in people who do not yet have it, while a therapeutic vaccine aims to help people who are already infected, and the two are tested against different goals. As of 2026, no HSV vaccine — prophylactic or therapeutic — has cleared Phase 3 to approval; a 2024 review described an effective HSV vaccine as still out of reach. [S5][S1]