Immune checkpoint inhibitor (ICI) treatment has produced striking results in patients with colorectal cancer (CRC) of the subtype deficient mismatch repair (dMMR). The majority of patients, however, have proficient MMR (pMMR) tumors, with limited effect of ICIs. The key difference between dMMR and pMMR tumors is the infiltration of cytotoxic T-cells. dMMR tumors have increased infiltration and thus increased efficacy from ICI treatment. The investigators conducted a proof of concept study where the investigators applied an intratumoral (IT) unaltered flu vaccine in ten patients with non-metastatic pMMR CRC. The intervention increased infiltration of cytotoxic T-cells and the immune checkpoint PD-L1, suggesting that IT flu vaccine primes pMMR tumors to ICI treatment. The investigators aim to test the combination of IT flu vaccine and ICI treatment in patients with non-metastatic pMMR CRC in a new trial. The hypothesis is that IT flu vaccine and ICI treatment will synergistically to induce cancer cell death.

Inclusion Criteria:

• Have histologically confirmed localized pMMR stage cT1N0M0 to cT4N2M0 (stage I to III) colorectal adenocarcinoma.

• Have indication for elective curative intended surgery without neoadjuvant therapy.

• Be ≥ 18 years of age on the date of signing the informed consent.

• Provide written informed consent

• Have Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

• Have adequate bone marrow function:

• Hemoglobin ≥ 6.2 mmol/L or ≥ 10 g/dL

• Absolute neutrophil count (ANC) ≥ 1.5 × 109/L

• Platelet count ≥ 100 × 109/L

• Have adequate kidney function defined as Glomerular filtration rate (GFR) ≥ 60 mL/min or creatinine ≤1.5 X upper limit of normal (ULN)

• Have adequate liver function defined as:

  • Total bilirubin ≤ 1.5 × ULN
  • Alanine aminotransferase (ALT): ≤ 2.5 × ULN
  • Alkaline phosphatase: ≤ 2.5 × ULN

• Follow the conditions regarding fertility, pregnancy, and lactation:

  o Female and male participants of reproductive potential (for definition refer to appendix 18) must agree to avoid becoming pregnant or impregnating a partner, respectively, while receiving pembrolizumab and for 180 days after the administration.

• Participants must use (or have their partner use) an acceptable method of contraception, as outlined in the appendix 16, during heterosexual activity, while receiving pembrolizumab and for 120 days after the administration.

• Women of reproductive potential (WORP) must have a negative serum or urine pregnancy test (minimum sensitivity 25 IU/L or equivalent units of HCG) within 72 hours prior to receiving pembrolizumab.

• Women must not be breastfeeding.

Exclusion Criteria:

• Has any serious or uncontrolled medical disorder that, in the opinion of the investigator or treating physician, may increase the risk associated with study participation or study drug administration, impair the ability of the subject to receive protocol therapy, or interfere with the interpretation of study results.
• Has an autoimmune disorder (except thyroiditis with replacement therapy and type I diabetes mellitus).
• Has received prior treatment with an anti-PD-1, anti-PD-L1, anti-PD-L2, anti-CD137, or anti-CTLA-4 antibody or any other antibody/drug specifically targeting T cell co-stimulation or checkpoint pathways.
• Has a known history of Human Immunodeficiency Virus (HIV) (HIV 1/2 antibodies), active chronic or acute Hepatitis B (e.g., HBsAg reactive) or Hepatitis C (e.g., HCV RNA is detected).
• Has a condition requiring systemic treatment with either corticosteroids (> 10 mg daily prednisone equivalents) or other immunosuppressive medications within 14 days of study drug administration. Inhaled or topical steroids and adrenal replacement doses > 10 mg daily prednisone equivalents are permitted in the absence of active autoimmune disease.
• Has received live vaccines within 30 days prior to first dose trial treatment (Examples of live vaccines include, but are not limited to: measles, mumps, rubella, chickenpox,
• Has recently received yellow fever, rabies, BCG, and typhoid (oral) vaccine.
• Has a history of allergy to study drug components or a history of severe hypersensitivity reaction to any monoclonal antibody
• Any previous allergic reaction to influenza vaccine or constituents, egg and chicken proteins, neomycin, formaldehyde or octoxinol-9
• Acute febrile illness
• Acute infectious disease
• Highly inflamed gastrointestinal tissue which is ulcerated and bleeding