Post-vaccination infection is a rare but serious complication, particularly when involving resistant pathogens such as Methicillin-resistant Staphylococcus aureus (MRSA).^7,8 In this case, the infant developed pyomyositis and osteomyelitis at the site of the hepatitis B vaccine injection. Based on available medical records, the vaccination was administered at a primary healthcare center; however, detailed documentation regarding vaccination practices (skin antisepsis, injection technique, needle handling, and post-injection monitoring) was unavailable, precluding formal assessment of adherence to aseptic technique and representing an important limitation of this report. Although a direct causal relationship cannot be definitively established, the temporal association between intramuscular vaccination and subsequent invasive MRSA infection raises important considerations regarding injection safety practices in infants.

From a pathophysiological perspective, a biologically plausible pathway may explain this association, whereby injection-related microtrauma facilitates inoculation of cutaneous flora into deeper tissues, particularly in infants with an immature immune system and underdeveloped skin barrier. While vaccination itself is not causative, breaches in aseptic technique during intramuscular injection may increase the risk of bacterial entry at the injection site.^8,9 This observation underscores the importance of strict adherence to aseptic injection practices, adequate training of primary healthcare workers, and vigilant surveillance of adverse events following immunization (AEFI).¹⁰ Strengthening these preventive measures aims to minimize the risk of rare yet serious post-injection complications while maintaining confidence in routine immunization programs.

In addition to limitations in vaccination practice documentation, detailed documentation of the early medical evaluations at the referring facilities was limited. Available information suggests that the patient received symptomatic treatment during the initial course of illness, without comprehensive sepsis evaluation appropriate for young infants or empiric intravenous antibiotic therapy. This delay in appropriate evaluation and treatment may have contributed to disease progression and deep tissue involvement. In addition, the lack of early antimicrobial coverage against MRSA could have facilitated ongoing infection. This represents an important limitation of the present report and underscores the need for strict adherence to fever protocols for neonates and young infants at primary healthcare facilities.

The susceptibility of infants to invasive MRSA infection is further influenced by host factors, including reduced neutrophil function, impaired phagocytic activity, and suboptimal pro-inflammatory cytokine responses, which collectively increase vulnerability to invasive infections.¹¹ In this case, both blood and pus cultures yielded MRSA, supporting the diagnosis of an invasive infection. Staphylococcus aureus possesses multiple virulence factors that facilitate adhesion to host tissues, invasion, and evasion of host immune defenses, enabling dissemination to deep soft tissue and bone. Invasive musculoskeletal infections may occur through hematogenous spread or direct inoculation and typically present with localized swelling, tenderness, and fever.¹²

The resistance profile of the isolate in this case was consistent with patterns commonly observed in community-associated MRSA (CA-MRSA), which typically exhibit resistance to β-lactam antibiotics while remaining susceptible to non-β-lactam agents such as vancomycin and linezolid.¹³ The occurrence of deep soft tissue and bone involvement in an otherwise healthy infant raises concern for a potentially virulent strain. Certain CA-MRSA lineages harbor virulence determinants, including Panton-Valentine leukocidin (PVL), which have been associated with severe skin and soft tissue infections and invasive musculoskeletal disease, even in patients without traditional risk factors.^14,15 However, molecular characterization and virulence factor profiling were not performed in this case, representing an important limitation of this report. Future studies incorporating molecular typing may help clarify the epidemiology and virulence profiles of invasive MRSA strains in early infancy.

Given the severity of infection in a young male infant and the involvement of catalase-producing Staphylococcus aureus, an underlying primary immunodeficiency, particularly chronic granulomatous disease (CGD), should be considered. Comprehensive immunologic evaluation was not performed due to resource limitations and prioritization of acute clinical management. Although there was no history of recurrent infections or family history suggestive of immunodeficiency, the absence of formal immunologic testing precludes definitive exclusion of CGD or other inborn errors of immunity and represents an important limitation of this report.

Although pyomyositis is more commonly encountered in tropical regions and predominantly affects young children aged 2-5 years, its occurrence in early infancy is rare. In this case, MRSA was isolated from both aspirated abscess material and blood cultures, confirming invasive musculoskeletal infection. Procedure-related factors during intramuscular injection may have facilitated bacterial entry into deeper tissues, particularly if microtrauma or breaches in aseptic technique occurred.^16,17

Infant osteomyelitis has unique features, including rapid spread from the metaphysis to the epiphysis because of vascular anastomoses in infants. Infection may extend between bone and nearby muscle tissues, as seen in this case, where both areas were affected.¹⁸ Managing MRSA infections requires proper antibiotic treatment. Empirically, infants suspected of having severe Staphylococcus aureus infections are often given intravenous vancomycin, especially when MRSA is a concern.¹⁹ In this case, the patient received vancomycin guided by susceptibility testing.

Vancomycin is frequently used to treat invasive MRSA infections in children, but its administration requires careful monitoring due to risks of nephrotoxicity and treatment failure linked to subtherapeutic levels. In cases of osteomyelitis and pyomyositis, especially in infants, treatment is often extended over several weeks, depending on the severity and clinical response. Monitoring clinical signs and laboratory markers, such as C-reactive protein, is crucial for evaluating treatment effectiveness. Along with antimicrobial therapy, surgical drainage or aspiration is often necessary to manage abscesses and obtain microbiological samples for targeted treatment.²⁰

Complications of invasive MRSA infections in infants may include sepsis and long-term musculoskeletal sequelae; therefore, early recognition and aggressive, targeted management are crucial to improve clinical outcomes.²¹ In the present case, timely referral and appropriate antimicrobial therapy after culture confirmation may have contributed to the favorable clinical response. This report highlights the importance of maintaining a high index of suspicion for deep soft tissue and musculoskeletal infection in young infants presenting with localized swelling and fever following intramuscular procedures, to facilitate prompt diagnostic evaluation and intervention. Systematic reporting of rare but serious post-injection infections is essential to improve clinical awareness and inform preventive strategies, while reinforcing the overall safety and public health value of routine immunization programs.²²