From the Counsellor's Chair

Could Personalised Cancer Vaccines Change What Happens After Cancer Surgery?

For decades, one of the most difficult problems in oncology has been what happens after the surgeon has successfully removed a cancer.

The scans may show no detectable disease. The tumour may be completely removed. The pathology may suggest that the margins are clear.

And yet, the oncologist may still recommend chemotherapy, radiotherapy, hormone therapy or immunotherapy.

Why? Because “no detectable cancer” does not necessarily mean “no cancer cells remain.”

Microscopic cancer cells may have already escaped the original tumour and remain somewhere in the body at levels too small to be detected by conventional imaging. These residual cells can eventually grow into a recurrence or metastasis.

This is where a remarkable development involving Moderna and Merck could potentially change the way we think about cancer treatment.

Their personalised mRNA therapy, intismeran autogene (formerly mRNA-4157/V940), has now produced positive Phase 3 results in patients with high-risk melanoma whose tumours had been surgically removed. The therapy is being tested in combination with Merck's immunotherapy Keytruda (pembrolizumab).

The important question is not simply whether this is a “cancer vaccine.” The more important question is: Could we eventually use a patient's own tumour to create a personalised treatment designed specifically to prevent that particular cancer from coming back?

The problem with cancer surgery

Surgery is one of the most powerful weapons against solid tumours.

A surgeon can remove the visible tumour and surrounding tissue, and pathology can confirm that the surgical margins are clear.

But cancer is not always confined to the mass that can be seen.

A patient can therefore be classified as having no evidence of disease (NED) while still carrying microscopic residual disease.

This creates a difficult clinical problem. If doctors treat everyone aggressively after surgery, some patients may receive treatments they ultimately did not need. If they don't treat aggressively enough, some patients whose cancer has already begun spreading microscopically may later develop a recurrence.

This is the rationale behind adjuvant therapy — treatment given after the primary cancer has been removed to reduce the probability of recurrence.

The Moderna/Merck approach potentially adds another layer: instead of giving the immune system a relatively broad instruction to attack cancer, what if we could give it a highly specific molecular description of that patient's cancer?

How does the personalised cancer vaccine work?

The word “vaccine” can be slightly misleading.

This isn't a conventional preventative vaccine such as those used against HPV. It is a therapeutic cancer vaccine.

The patient has already had cancer. The objective is to train their immune system to recognise and attack cancer cells that may remain after surgery.

Step 1: Remove the tumour

The patient first undergoes surgery. A sample of the tumour is analysed genetically.

Scientists compare the tumour's genetic sequence with normal tissue. They are looking for mutations that are present in the cancer but not in the patient's healthy cells.

These tumour-specific mutations can create abnormal proteins called neoantigens.

Step 2: Identify the cancer's “fingerprints”

Cancer cells accumulate mutations. Some of those mutations result in abnormal proteins or protein fragments that can potentially be recognised by the immune system. These are called neoantigens.

Think of them as molecular fingerprints of the tumour.

The crucial advantage is that the fingerprints are specific to that patient's cancer.

Instead of saying, “This is what melanoma generally looks like,” the treatment is effectively trying to tell the immune system: “These are the particular molecular features of your tumour. Learn to recognise them.”

The Moderna/Merck vaccine is designed around multiple tumour-specific neoantigens identified from each patient's cancer.

Step 3: The patient's vaccine is manufactured

Scientists use the tumour's genetic information to design an individualised mRNA sequence.

The mRNA contains temporary instructions for producing selected tumour-associated neoantigens. The resulting treatment is therefore personalised to the individual patient rather than being one identical vaccine manufactured for everybody.

The mRNA is packaged so that it can enter cells.

Step 4: The mRNA gives immune cells a lesson

Once administered, the mRNA is taken up by cells and used as temporary instructions to produce the selected antigenic targets.

The immune system then encounters these targets. This activates an immune response involving T cells, particularly cytotoxic T cells capable of recognising cells displaying the corresponding tumour-derived peptides.

In simplified terms:

Tumour → genetic sequencing → identify neoantigens → design mRNA → immune system learns neoantigens → T cells seek cells displaying those targets.

The mRNA itself does not permanently alter the patient's DNA. It functions as temporary biological instructions.

But there is another important part: Keytruda

The vaccine isn't being used alone. It is being combined with pembrolizumab (Keytruda).

Cancer has evolved sophisticated mechanisms for suppressing the immune response. One of those mechanisms involves immune checkpoints.

PD-1 is an inhibitory receptor on T cells. Tumours can exploit the PD-1/PD-L1 pathway to effectively put the brakes on T-cell activity.

Pembrolizumab blocks PD-1.

In simple terms:

Vaccine: “Here is what the cancer looks like.”

Keytruda: “Now remove one of the mechanisms preventing the immune system from attacking it.”

This creates a potentially powerful combination: the vaccine provides the target, while Keytruda helps release the brakes.

What have the trials actually shown?

The earlier Phase 2b KEYNOTE-942 study followed patients with high-risk melanoma after complete surgical resection.

At five years, the combination of intismeran and pembrolizumab produced a 49% reduction in the risk of recurrence or death compared with pembrolizumab alone.

It also produced a 59% reduction in the risk of distant metastasis or death.

That durability is important. Cancer recurrence isn't something you can adequately assess after a few months. The fact that the benefit remained evident at five years makes the earlier findings considerably more interesting.

Then came the Phase 3 trial

The Phase 3 INTerpath-001 trial enrolled more than 1,100 patients with completely resected high-risk stage IIB-IV melanoma.

Patients received either intismeran + Keytruda or Keytruda alone.

The trial met its primary endpoint of recurrence-free survival and also met the key secondary endpoint of distant metastasis-free survival.

That is a major distinction from an interesting early-stage laboratory finding. This is now Phase 3 evidence from a large randomised clinical trial.

However, there is an important qualification. The companies have announced positive topline results, but the complete detailed dataset has not yet been published. The full numerical results, including mature overall-survival information, are still awaited.

So it is reasonable to call this a major breakthrough. It is not yet reasonable to say that cancer vaccines have been proven to cure cancer.

Could this change the standard cancer treatment pathway?

Potentially, yes.

The conventional pathway is:

Cancer diagnosed → surgery → pathology → determine recurrence risk → adjuvant treatment → surveillance

The future could potentially look more like:

Cancer diagnosed → surgery → tumour sequencing → identify recurrence risk → manufacture personalised vaccine → immune priming → checkpoint inhibition → surveillance

That would represent a significant shift toward precision oncology.

Instead of asking only, “What type of cancer does this patient have?” doctors could increasingly ask, “What specifically makes this patient's cancer biologically different from everyone else's?”

The biggest potential change: treating microscopic disease

This may ultimately be more important than treating an established tumour.

Once a cancer has produced a large metastatic tumour, treatment becomes substantially more difficult. But immediately after surgery, there may only be a tiny number of residual malignant cells.

That creates an intriguing therapeutic window.

The cancer has been physically reduced to almost nothing. Now the immune system could potentially be trained to find what the surgeon cannot see.

In other words:

Surgery removes the visible disease.

The personalised vaccine could potentially help the immune system hunt the invisible disease.

That is a fundamentally different way of thinking about adjuvant cancer treatment.

And this could extend far beyond melanoma

Melanoma is currently the clearest demonstration of the concept.

But Moderna and Merck are investigating intismeran in other cancers, including non-small-cell lung cancer, and the broader programme is exploring additional solid tumours.

If similar results are eventually demonstrated in other cancers, the implications could be enormous.

Imagine someone having a lung tumour removed. The pathology indicates a high probability of recurrence. Instead of relying solely on conventional adjuvant therapy, the tumour could potentially be sequenced and used to create a personalised immune treatment specifically targeting that patient's cancer.

The same conceptual framework could potentially be explored in cancers such as bladder, kidney, pancreatic, gastrointestinal and other solid tumours.

But this remains an area of active research. Success in melanoma does not automatically mean the same strategy will work equally well in every cancer.

Why this could be a paradigm shift

Traditional oncology has often relied on population-level treatment. If you have a particular cancer at a particular stage, there are established treatment protocols based on what has worked best across thousands of patients.

Precision medicine has been gradually changing this.

Genomic testing can identify mutations that make a particular tumour susceptible to particular drugs.

But a personalised mRNA cancer vaccine takes the concept further. The treatment itself can potentially be manufactured around the unique molecular characteristics of the individual patient's tumour.

Traditional approach: One cancer type → one treatment strategy.

Precision-medicine approach: One cancer subtype → treatment based on molecular characteristics.

Personalised cancer vaccine approach: One patient's tumour → one personalised immune target profile.

That is the real significance of this technology.

But there are still major questions

1. Does preventing recurrence translate into longer overall survival?

Recurrence-free survival and overall survival are not the same endpoint. We need mature Phase 3 survival data.

2. Will it work across different cancers?

Melanoma has particular biological characteristics and is relatively immunogenic. Other cancers may be much harder for the immune system to recognise.

3. How quickly can personalised vaccines be manufactured?

A personalised cancer vaccine potentially requires tumour removal, sequencing, computational analysis, vaccine design, manufacturing, quality control and administration.

4. How much will it cost?

Personalised manufacturing is inherently more complicated than producing one mass-market drug. Healthcare systems will have to determine whether the reduction in recurrence justifies the cost.

5. Who actually needs it?

The ideal candidates may be patients who have had their cancer completely removed but have a sufficiently high probability of microscopic residual disease. That raises another important question: Can we accurately identify the patients most likely to benefit?

The bigger picture

The most exciting possibility isn't necessarily that we will replace surgery, chemotherapy or immunotherapy. It may be that we reorganise when and how we use them.

Cancer treatment could increasingly become a coordinated sequence:

Remove the tumour.

Decode the tumour.

Identify its vulnerabilities.

Train the immune system against those vulnerabilities.

Remove immune-system brakes where appropriate.

Monitor for molecular evidence of recurrence.

Intervene before a visible tumour develops.

That would represent a major conceptual shift.

Rather than waiting for cancer to return and then trying to treat it again, oncology could increasingly move toward anticipating recurrence and attempting to eliminate microscopic disease before it becomes clinically detectable.

The bottom line

The Moderna/Merck cancer vaccine is not a universal “cure for cancer.”

But something important has happened.

For the first time, a personalised mRNA cancer vaccine has produced positive Phase 3 results in a large randomised trial, when combined with Keytruda, in patients whose high-risk melanoma had been surgically removed.

The earlier five-year data provide additional evidence that the benefit can be durable, with a 49% reduction in recurrence or death and a 59% reduction in distant metastasis or death in the Phase 2b study.

The real breakthrough may therefore not be “we have invented a vaccine that kills cancer.”

It is potentially much more sophisticated:

“We may be learning how to turn a patient's own tumour into a blueprint for teaching their immune system to prevent that cancer from returning.”

If this approach succeeds across multiple cancer types, the period immediately following successful cancer surgery could become one of the most important windows in the entire treatment journey.

And that could fundamentally change the question doctors ask after removing a cancer.

Instead of simply asking: “Have we removed the tumour?”

the question may increasingly become:

“Now that we've removed it, how do we teach the immune system to make sure it doesn't come back?”

Important medical note

This article discusses an investigational therapy and should not be interpreted as medical advice or as evidence that patients should alter current cancer treatment protocols. Regulatory approval and detailed Phase 3 results are still pending.