Herpevac / Simplirix

Simplirix was an experimental vaccine meant to stop women who had never had HSV from getting genital herpes. Herpevac was the name of its pivotal Phase 3 trial.

That trial missed its primary endpoint. Genital herpes disease from either HSV type occurred in 35 of 3798 vaccine recipients and 35 of 3076 controls, a vaccine efficacy of 20 percent (95% CI −29 to 50).

Protection against HSV-2, the type the vaccine's antigen came from, was not demonstrated. Efficacy was −38 percent against HSV-2 genital disease (95% CI −167 to 29) and −8 percent against HSV-2 infection (95% CI −59 to 26).

Efficacy against HSV-1 genital disease, a secondary analysis within the same trial, was 58 percent (95% CI 12 to 80). GlaxoSmithKline stopped worldwide development in September 2010 and nothing was approved. The result does not establish that HSV vaccines as a category cannot work.

Simplirix paired a truncated form of glycoprotein D from HSV-2 with an adjuvant, meaning a substance added to strengthen the immune response a vaccine produces. That adjuvant combined an aluminium salt with a bacterial-derived component, the pairing GSK labels AS04.

Herpevac is the trial's name and Simplirix the trade name GSK gave the product. GSK208141 and SB208141 are the sponsor's codes for a development sequence running from 1992 to 2007.

A separate vaccine combining HSV-2 glycoprotein B and glycoprotein D with a different adjuvant was a different program, with a different antigen and population. Nothing from it belongs here.

The program was preventive, meaning it aimed to stop genital herpes in people who did not already have HSV. Its antigen came from HSV-2, but the pivotal trial's prespecified primary endpoint counted genital herpes disease caused by HSV-1, HSV-2 or both.

The trial existed to test an earlier subgroup finding prospectively. Two earlier GSK Phase 3 studies, in people whose regular partners had genital herpes, were null overall: efficacy was 38 percent in the first (95% CI −18 to 68) and 42 percent among women in the second (95% CI −31 to 74).

Efficacy reached significance only in one subgroup, women seronegative for both HSV types, at 73 percent (95% CI 19 to 91) and 74 percent (95% CI 9 to 93). It was not efficacious in HSV-1-seropositive women or in men.

The two earlier studies, reported together in 2002, randomized 847 subjects seronegative for both HSV types and 1867 HSV-2-seronegative subjects, dosing at months 0, 1 and 6.

The Herpevac Trial for Women enrolled 8323 women aged 18 to 30 who were seronegative for both HSV types. Enrollment ran at 50 sites in the United States and Canada from January 2003 to November 2007, with three doses and follow-up to month 20.

The comparator was not a placebo. It was a reduced formulation of a licensed hepatitis A vaccine.

GSK was the registered lead sponsor, with no collaborator listed. NIAID supported the trial under contract, co-funded it, sponsored the independent monitoring board and co-authored the primary report. This was a GSK-sponsored and NIAID-supported trial rather than an NIAID-sponsored one.

The trial missed its primary endpoint. Genital herpes disease from either HSV type occurred in 35 of 3798 vaccine recipients and 35 of 3076 controls, a vaccine efficacy of 20 percent (95% CI −29 to 50).

Protection against HSV-2 was not demonstrated. Efficacy against HSV-2 genital disease was −38 percent (95% CI −167 to 29), at 24 cases against 14. Efficacy against HSV-2 infection with or without disease was −8 percent (95% CI −59 to 26), at 62 cases against 46.

Those point estimates are negative because more cases occurred among vaccine recipients. Every one of those intervals crosses zero and extends well into benefit, so the trial does not establish that the vaccine increased HSV-2 risk either.

Efficacy against HSV-1 was statistically significant. Against HSV-1 genital disease it was 58 percent (95% CI 12 to 80), at 11 cases against 21. Against HSV-1 infection with or without disease it was 35 percent (95% CI 13 to 52).

The 58 percent and 82 percent HSV-1 figures are the same trial analysed differently, not competing results. The 58 percent is two-dose efficacy against all HSV-1 genital disease; three-dose efficacy against all cases was 77 percent (95% CI 31 to 92), and three-dose efficacy restricted to culture-positive cases was 82 percent (95% CI 35 to 95).

Every type-specific estimate here is a secondary analysis in a trial that missed its primary endpoint. The p values were unadjusted for multiple testing and the case counts are small.

Injection-site reactions occurred in 88 percent of vaccine recipients against 72 percent of controls. Serious adverse events occurred in 5 percent of each group. GSK called the safety profile acceptable, which is the sponsor's characterization, and a 20-month trial cannot establish long-term or rare-event safety.

The program generated large confirmatory evidence, meaning a Phase 3 field trial of 8323 people built to answer an efficacy question at scale. That evidence was negative for the primary endpoint. Evidence maturity describes the study design and the question answered, not whether the candidate worked.

The Herpevac trial recruited precisely the subgroup in which the earlier studies had found efficacy, and tested it as a whole population rather than as a subgroup. The earlier benefit was not reproduced. That reading is HSVHQ's, and it does not identify which of the two settings gave the misleading answer.

The result does not establish that HSV vaccines can never work. One adjuvanted glycoprotein-D subunit vaccine missing its endpoint in one population over 20 months is not evidence about mRNA, live-attenuated, therapeutic or vector-based approaches. The investigators themselves wrote that approaches such as live attenuated vaccines or vaccine vectors may be required.

The HSV-1 result equally does not mean a working HSV-1 vaccine existed and was abandoned. It came from a secondary, multiplicity-unadjusted analysis with small case counts, protection against oral HSV-1 disease was not demonstrated, and nothing was licensed.

The program is discontinued. On September 29, 2010 GSK stated that it had decided not to pursue further worldwide development of Simplirix, following receipt of the Herpevac results, which it said showed an acceptable safety profile but did not meet the primary endpoint.

GSK gave no reason beyond the unmet primary endpoint and no breakdown by HSV type. The type-specific results were not published until January 2012, so the decision cannot have been a judgment on the HSV-1 finding as published.

Trial completion and program discontinuation are separate facts, and both hold here. The vaccine was never approved, the registry record shows no expanded-access route, and no later study of this vaccine was identified in the sources checked through September 8, 2026.

No further development of this vaccine was identified, and the developer stated in 2010 that worldwide development would not continue. The program itself has no confirmed next step.

This record would warrant an update if the 2002 report's full text establishes whether the subgroup analysis was prespecified, or if any published analysis establishes why protection against HSV-2 was absent.