A vaccine used to prevent meningococcal disease does not appear to reduce gonorrhoea infections among gay and bisexual men at high risk of acquiring the sexually transmitted infection, according to a large Australian clinical trial, published in NEJM.

The finding matters because some health programmes have begun offering the meningococcal B vaccine, known as 4CMenB, to people at increased risk of gonorrhoea. Those programmes were based largely on earlier observational studies that suggested the vaccine might provide partial protection.

However, the new randomised trial found no reduction in gonorrhoea incidence among its participants. It adds to results from two other clinical trials that have also not found clear evidence that 4CMenB prevents gonorrhoea in this group.

Why researchers thought the vaccine might help

Gonorrhoea is a common sexually transmitted infection caused by the bacterium Neisseria gonorrhoeae. It can infect the genitals, rectum and throat, and may cause no symptoms, particularly at throat and rectal sites.

When untreated, gonorrhoea can lead to complications including pelvic inflammatory disease, infertility and an increased risk of acquiring or transmitting HIV. Rising resistance to antibiotics used to treat the infection has also made prevention increasingly important.

The 4CMenB vaccine was developed to protect against meningococcal B disease, a rare but potentially life-threatening infection caused by a related bacterium, Neisseria meningitidis. Because the two bacteria share some genetic and surface features, scientists considered it biologically plausible that a vaccine against meningococcal B could also trigger immune responses that offered some protection against gonorrhoea.

Several observational studies appeared to support that possibility. These studies compared vaccination histories among people diagnosed with gonorrhoea and people diagnosed with other sexually transmitted infections, such as chlamydia. Some suggested that people with gonorrhoea were less likely to have received 4CMenB.

But observational studies cannot reliably account for every difference between vaccinated and unvaccinated people, including differences in sexual health service use, testing patterns or other prevention behaviours.

What the new trial found

The GoGoVax study was a randomised controlled trial. It involved gay and bisexual men with a recent history of gonorrhoea or syphilis, who were selected because they had a relatively high likelihood of acquiring gonorrhoea during the study period.

Participants were randomly assigned to receive either 4CMenB or a placebo. A placebo is an inactive injection used for comparison, allowing researchers to assess whether any differences in outcomes are likely to be due to the vaccine rather than other factors.

The researchers found that vaccination with 4CMenB had no effect on the incidence of gonorrhoea in the study population.

In plain language, men who received the meningococcal B vaccine were not less likely to be diagnosed with gonorrhoea than men who received placebo.

The result is consistent with two other randomised trials, DOXYVAC and MenGO, which also reported no protective effect of 4CMenB against gonorrhoea among men at high risk of infection.

Why randomised trials can produce different results

Randomised controlled trials are designed to provide a strong test of whether an intervention causes a particular outcome.

Because participants are assigned by chance to receive the vaccine or placebo, the groups should be broadly similar at the start of the study. This helps reduce the influence of confounding factors: differences other than vaccination that could affect a person’s risk of infection.

For example, people who choose to be vaccinated may differ from those who do not in ways that are difficult to measure. They may attend sexual health services more often, test more frequently, use other prevention measures differently or have different patterns of sexual contact. Any of these factors could make a vaccine appear more or less effective in an observational study.

In GoGoVax, randomisation appears to have produced balanced groups, making it less likely that such differences explain the findings.

How strong is the evidence?

The evidence is strong for the specific group studied: gay and bisexual men with a recent history of gonorrhoea or syphilis, and therefore a high risk of further infection.

The trial does not show that 4CMenB could never have any effect against gonorrhoea in any population. Its findings cannot automatically be applied to women, heterosexual men, adolescents or people with a lower risk of gonorrhoea.

The researchers noted that the participants’ high frequency of previous gonorrhoea may be relevant. It is possible, though unproven, that people with repeated exposure to the bacterium respond differently from other groups, or that any small vaccine effect would be difficult to detect where infection risk is particularly high.

Even so, the consistency of results across three randomised trials means the earlier promise suggested by observational studies has not been borne out for high-risk men who have sex with men.

What this means for the public

For people who have received or intend to receive 4CMenB in the hope of preventing gonorrhoea, the study provides an important clarification. The vaccine should not be relied upon for protection against this sexually transmitted infection.

This does not affect its approved use against meningococcal B disease. The vaccine remains an established and effective option for preventing meningococcal disease in groups for whom it is recommended.

Current gonorrhoea prevention advice remains unchanged. Condoms can reduce the risk of transmission, although they do not provide complete protection for infections spread through oral sex or contact with areas not covered by a condom. Regular sexual health screening is also important because gonorrhoea often causes no symptoms.

People with symptoms, a recent sexual contact diagnosed with gonorrhoea, or concerns about exposure should contact a sexual health clinic or other appropriate healthcare service.

Implications for vaccination programmes

The findings may have implications for programmes that have introduced 4CMenB vaccination specifically to reduce gonorrhoea.

The UK began offering the meningococcal B vaccine to gay and bisexual men at greatest risk of gonorrhoea in 2025. Galicia in Spain also introduced a programme for adults at increased risk. Both were informed by observational evidence suggesting possible cross-protection.

The GoGoVax investigators said the new results, alongside the other randomised trials, do not support using 4CMenB as a gonorrhoea prevention measure for high-risk gay and bisexual men.

Decisions about existing programmes will need to consider the full evidence, including surveillance data collected after implementation.

What researchers still need to find out

Trials in other groups, including women and people at lower risk of gonorrhoea, are under way, with results expected within the next year.

These studies may help answer whether the vaccine has any role in different populations. They may also shed light on why observational studies suggested protection while randomised trials in high-risk men have not.

More broadly, the need for an effective gonorrhoea vaccine remains pressing. Antibiotic resistance continues to limit treatment options, and no vaccine is currently approved specifically to prevent gonorrhoea.

The wider picture

The new evidence is disappointing for efforts to find an additional tool against gonorrhoea, but it offers a clearer basis for public health decisions.

For now, 4CMenB remains valuable for preventing meningococcal B disease, but the best available randomised evidence indicates that it does not prevent gonorrhoea among gay and bisexual men at high risk of infection.

Prevention will continue to depend on accessible testing, prompt treatment, partner notification and safer-sex measures while research into a dedicated gonorrhoea vaccine continues.