Vaccine-related reaction: The recent administration of the Shingrix vaccine 10 days ago could be related to the patient's symptoms. The Shingrix vaccine is known to cause systemic reactions such as fever, headache, and malaise, especially in older adults. The timing and nature of the symptoms align with a vaccine reaction.

Influenza or other viral infections not detected by the viral panel: Despite a negative viral panel, it's possible that the patient has a viral infection not covered by the panel or an infection that was not detected due to the timing of the test or the quality of the sample.
Bacterial infection with atypical presentation: The absence of leukocytosis and negative imaging studies does not rule out a bacterial infection, especially in an elderly patient who may have an atypical presentation.
Autoimmune or inflammatory condition: Conditions like giant cell arteritis or polymyalgia rheumatica could present with fever, headache, and malaise, especially in an older adult.

Endocarditis: Although less likely given the lack of specific cardiac symptoms and normal diagnostic tests, endocarditis can present subtly and is a diagnosis that could be deadly if missed.
Abdominal or pelvic abscess: Despite a normal CT abdomen and pelvis, an abscess could be small or in an atypical location, not visible on initial imaging. Repeated imaging or further investigation might be necessary if symptoms persist.
Central nervous system infection: Meningitis or encephalitis could present with headache and fever. A lumbar puncture might be necessary to rule out these conditions, especially if the headache worsens or if there are changes in mental status.

Lymphoma or other hematologic malignancies: These conditions can cause persistent fever and malaise. Although rare, they are important to consider in the differential diagnosis, especially if other causes are ruled out.
Factitious fever: This is a condition where a patient intentionally produces or feigns symptoms of illness. It's rare but should be considered if there's a high index of suspicion based on the patient's behavior or inconsistencies in the history.
Rare infections (e.g., brucellosis, Q fever): Depending on the patient's exposure history (e.g., occupational exposure to animals), rare infections could be considered, especially if common causes are ruled out.