HSV529

What it is

HSV529 is a whole-virus HSV-2 vaccine that cannot replicate. It was derived from a laboratory strain called dl5-29, from which two genes the virus needs to copy itself (UL5 and UL29) were deleted. The idea is to show the immune system many HSV-2 proteins at once, rather than the one or two used by protein-subunit vaccines, without the risk of a virus that can spread. In mice and guinea pigs it raised neutralizing antibodies and reduced genital disease after HSV-2 challenge, and it did not replicate in sensitive animal tests — animal results that do not, on their own, show protection in people.

Where it stands

HSV529 has been studied in three Phase 1 trials, which test safety and immune response rather than whether a vaccine prevents or treats disease:

• NIAID trial (NCT01915212), completed. A randomized, placebo-controlled study in 60 adults split into three groups by prior HSV-1/HSV-2 infection. The vaccine was considered safe; injection-site reactions were more common than with placebo. Among vaccinated people who had never had either virus, 78% developed at least a four-fold rise in neutralizing antibody after three doses; people already infected showed no such rise. T-cell responses were described as modest.
• Fred Hutchinson trial (NCT02571166), completed. A small study in adults already living with HSV-2 — a therapeutic question — looking at immune responses in the genital skin.
• Sanofi trial (NCT04222985), terminated. The registry record lists numbered HSV-2 vaccine formulations without naming them. A 2025 paper reports a matching trial — 24 adults with recurrent genital herpes — that tested HSV529 alongside G103, a three-protein HSV-2 vaccine, and reported acceptable safety, but it gives no registry number, so the match is likely rather than confirmed. The registry records that the trial was stopped for business reasons and its planned second part was not run.

No efficacy trial of HSV529 has been run, and ClinicalTrials.gov lists no active HSV529 trial. That is not the same as a formal announcement that the program has ended — we have not found one — so we describe where the trials stand rather than label it discontinued.

What this does and doesn’t mean

HSV529 is tested both as a potential preventive vaccine (in people never infected) and as a therapeutic one (in people who already have HSV-2), and the evidence differs: the antibody response seen in uninfected volunteers did not appear in people already infected. A good immune response in a Phase 1 trial is not evidence that a vaccine prevents infection or reduces outbreaks.