Bromhexine Prophylaxis and Pharmacologically Attenuated Natural Immunization: A New Paradigm of Host-Directed Modulation of SARS-CoV-2 and Influenza Virus Infections

1. Introduction

Recent comprehensive reviews identify TMPRSS2 as a central host dependency factor exploited by both SARS-CoV-2 and influenza A viruses and highlight its potential as a broad-spectrum host-directed antiviral target. Importantly, these reviews also emphasize that TMPRSS2 inhibition does not necessarily abolish viral entry, as alternative protease-dependent pathways remain available. This provides a mechanistic rationale for strategies aimed at attenuating, rather than completely preventing, viral infection [1,2,3].

Bromhexine hydrochloride (BRH) is a functional inhibitor of the host serine protease TMPRSS2, a key mediator of viral entry into respiratory epithelial cells for both SARS-CoV-2 and influenza viruses [4,5,6]. Although BRH has shown promise in the prevention and treatment of COVID-19, clinical outcomes have been inconsistent [14,15,16], and clinical evidence for influenza remains limited [1]. We propose that these apparently conflicting findings can largely be explained by two critical determinants: the timing of administration and the route of drug delivery [1,7,8].

Because TMPRSS2 functions at the earliest stage of infection by facilitating viral entry, its inhibition is expected to be most effective before or immediately following viral exposure. Accordingly, BRH is most likely to provide maximal benefit when administered as continuous prophylaxis during periods of increased viral circulation or as early post-exposure prophylaxis. Once infection is established and active viral replication is underway, the capacity of bromhexine to modify disease progression is expected to decline substantially. Nevertheless, inhaled bromhexine may still reduce local viral spread by inhibiting TMPRSS2 within the respiratory epithelium while simultaneously exerting anti-inflammatory and potentially direct antiviral effects [8].

This concept is supported by the pharmacokinetic properties of BRH. Following oral administration, bromhexine accumulates in pulmonary tissues, achieving concentrations approximately 2.4- to 5.9-fold higher than those in plasma, with bronchial tissue concentrations ranging from 1.5- to 3.2-fold above plasma levels. These tissue concentrations are considered sufficient to achieve pharmacologically relevant inhibition of TMPRSS2 [9].

Consistent with this hypothesis, we recently analyzed the outcomes of 125 individuals who received prophylactic bromhexine during the COVID-19 pandemic [8]. Continuous prophylactic administration throughout periods of high viral transmission was associated with a marked reduction in the incidence of COVID-19, while breakthrough infections were generally mild. In addition, we reported a retrospective study evaluating bromhexine prophylaxis during the 2024/2025 influenza epidemic in Bulgaria, in which the incidence of influenza was reduced by approximately 3.5-fold [7]. Population-based modeling further suggested that widespread prophylactic use of bromhexine could provide substantial healthcare and economic benefits [10].

The observation that TMPRSS2 inhibition does not completely prevent viral infection, but rather markedly reduces viral entry and disease severity while preserving sufficient antigen exposure for the development of adaptive immunity, prompted us to propose the concept of Pharmacologically Attenuated Natural Immunization (PANI). In the present report, we describe several illustrative clinical cases supporting this concept and discuss its potential implications for the prevention of COVID-19, influenza, and other TMPRSS2-dependent respiratory viral infections.

2. Index Case: SARS-CoV-2 Infection with Minimal Clinical Expression and Durable Humoral Immunity Following Bromhexine Prophylaxis

The absence of any SARS-CoV-2 vaccination after 2021 makes the observed serological profile particularly noteworthy, as the measured anti-spike (S) IgG response cannot be attributed to a recent vaccine-induced booster effect. Instead, the antibody profile is most consistent with natural SARS-CoV-2 infection acquired in October 2025.

At the time of SARS-CoV-2 exposure, the 72-year-old male individual (BMI, 35.3 kg/m²) had been receiving bromhexine hydrochloride prophylactically (8 mg twice daily) for approximately two weeks. Bromhexine has been proposed to inhibit the host serine protease TMPRSS2, a key enzyme mediating the entry of both SARS-CoV-2 and influenza viruses into respiratory epithelial cells through proteolytic activation of their fusion proteins [1]. Although TMPRSS2 inhibition may effectively block the TMPRSS2-dependent entry pathway, it is unlikely to completely prevent infection because alternative entry mechanisms remain available. Consequently, bromhexine is expected to reduce the efficiency of viral entry and early viral replication rather than abolish infection altogether.

Consistent with this mechanism, the patient remained essentially asymptomatic, reporting only mild throat irritation, as previously described [11]. SARS-CoV-2 infection was detected incidentally by a rapid antigen test rather than because of clinically apparent COVID-19. This clinical course is compatible with the hypothesis that bromhexine prophylaxis markedly reduced the initial viral burden while permitting limited viral replication through alternative entry pathways. Such low-level antigen exposure may nevertheless have been sufficient to induce a robust adaptive immune response.

The subsequent humoral immune response was remarkable. Approximately three months after infection (January 2026), the quantitative anti-spike antibody concentration reached 2,080 BAU/mL, indicating a strong humoral response. Approximately nine months after infection (July 2026), anti-spike IgG remained strongly positive, with a Vircell anti-S IgG ratio of 22.209. Although WHO-standardized quantitative antibody concentrations (BAU/mL) cannot be directly compared with assay-specific semi-quantitative indices such as the Vircell IgG ratio, both measurements consistently indicate a vigorous and sustained anti-spike immune response.

Following natural SARS-CoV-2 infection, antibody responses typically exhibit biphasic kinetics, with a relatively rapid decline during the first 3–6 months followed by a slower decay phase maintained by long-lived plasma cells and memory B cells [12]. Within this context, the persistence of strong anti-spike IgG reactivity nine months after infection suggests well-preserved humoral immune memory. This finding is particularly noteworthy given the patient's age (72 years), as immunosenescence is generally associated with diminished antibody responses and more rapid waning of humoral immunity following infection.

The present observation should be interpreted in the context of our previous studies investigating bromhexine prophylaxis [7,8,11,13. Collectively, these studies suggest that pharmacological inhibition of TMPRSS2 may attenuate viral entry sufficiently to reduce clinical disease while preserving adequate antigen exposure for adaptive immune priming.

Taken together, these findings support the concept that pharmacological attenuation of viral entry, rather than complete prevention of infection, may preserve the benefits of natural immune priming while substantially reducing disease severity. We propose the concept of Pharmacologically Attenuated Natural Immunization (PANI), whereby transient pharmacological attenuation of viral entry permits controlled natural antigen exposure sufficient to elicit durable adaptive immunity while minimizing clinical disease. Although this concept is currently supported primarily by observational evidence and biological plausibility, it provides a mechanistically grounded and clinically testable framework for future prospective studies evaluating host-directed prophylactic strategies against TMPRSS2-dependent respiratory viral infections, including SARS-CoV-2 and influenza viruses.

3. Paired Household Observation: High-Risk Individual with Absent Clinical Disease and Persistent Anti-Spike Immune Response

An additional observation from the same household provides further context for this hypothesis. The patient considered the possibility that he had transmitted SARS-CoV-2 infection to his wife, although she did not develop any clinical symptoms suggestive of COVID-19.

His wife was 71 years old, weighed 30 kg, and was 159 cm tall (body mass index [BMI], 11.9 kg/m²). She represented a high-risk host profile, with chronic obstructive pulmonary disease (COPD), severe underweight, previous pancreatic cancer treated with pancreaticoduodenectomy (Whipple procedure) three years earlier, and a long-standing history of heavy cigarette smoking (approximately 30 cigarettes per day).

Because of her chronic respiratory disease and increased vulnerability to respiratory viral infections, she had been taking bromhexine hydrochloride continuously as prophylaxis (8 mg twice daily) against COVID-19 and influenza. Approximately three months after the presumed household exposure, serological testing demonstrated an anti-SARS-CoV-2 spike antibody concentration of 1,040 BAU/mL. At approximately nine months after exposure, repeat testing showed persistent anti-spike IgG reactivity, with a Vircell anti-S IgG ratio of 3.43.

Interpretation of these findings requires consideration of her previous SARS-CoV-2 vaccination in early 2021. Therefore, the detected anti-spike antibodies cannot be attributed exclusively to natural infection, as residual vaccine-induced immune memory may have contributed to the observed response. However, the prolonged interval since vaccination, the close household exposure, the absence of clinical disease, and the persistence of measurable anti-spike antibodies are compatible with either a clinically silent SARS-CoV-2 infection, an anamnestic immune response triggered by natural antigen exposure, or a combination of both mechanisms.

Together, these paired household observations are of particular interest. The husband, without SARS-CoV-2 vaccination after 2021, experienced an incidentally detected infection with minimal symptoms and subsequently developed a strong and durable anti-spike antibody response. His wife, despite multiple risk factors for severe COVID-19—including advanced age, COPD, severe underweight, previous pancreatic malignancy, major abdominal surgery, and heavy smoking history—remained clinically asymptomatic following presumed exposure while maintaining measurable anti-spike immune reactivity.

4. Additional Observations Supporting the Hypothesis

Beyond the paired household observation described above, additional real-world observations from individuals receiving long-term bromhexine hydrochloride (BRH) prophylaxis provide further context for the proposed concept of pharmacologically attenuated natural immunization and the potential role of host-directed modulation of respiratory viral infections.

Previously, we described a 77-year-old vaccinated woman with multiple significant comorbidities, including breast malignancy, chronic kidney disease, pulmonary hypertension, rheumatoid arthritis, Hashimoto’s thyroiditis, Sjögren syndrome, and other chronic conditions. She received BRH prophylaxis (3 × 1 tablets daily) for six months. In March 2022, she developed SARS-CoV-2 infection; however, the clinical course was remarkably mild, consisting only of slight throat and nasal irritation. Without specific antiviral treatment, her rapid antigen test became negative after one week. In contrast, her daughter and son-in-law, both vaccinated but not receiving BRH prophylaxis, developed COVID-19 during the same period and experienced severe disease [11].

Importantly, this patient continued seasonal BRH prophylaxis thereafter, taking bromhexine for at least six months annually. During subsequent follow-up, she has not experienced clinically apparent COVID-19 or influenza despite her advanced age and substantial comorbidity burden.

A second previously described observation involved an 88-year-old unvaccinated woman with hypertension and type II diabetes mellitus. She received BRH prophylaxis (3 × 1 tablets daily) for approximately four months annually and remained free of clinically diagnosed COVID-19 despite direct household exposure to infected family members. At the age of 92 years, she continues seasonal BRH prophylaxis and has not developed clinically apparent COVID-19 or influenza [11].

A further observation was obtained during the 2025/2026 influenza season. An 8-year-old child with a history of frequent respiratory infections received BRH prophylaxis for approximately five months. During a period of increased influenza circulation, the child remained free of influenza, in contrast to many classmates who developed symptomatic infection.

Although these observations do not establish prophylactic efficacy in the absence of controlled studies, they are notable because they involve individuals from markedly different age groups and clinical backgrounds, including very elderly patients with multiple comorbidities and a child at increased risk of respiratory infections. The consistent absence or marked attenuation of clinically apparent respiratory viral disease during periods of potential exposure provides additional hypothesis-generating support for the possibility that BRH may influence the clinical expression of respiratory viral infections through host-directed mechanisms.

5. Long-Term Prophylaxis Observations: Tolerability and Absence of Clinically Apparent Respiratory Viral Infections

Additional real-world observations provide further context regarding both the long-term tolerability of bromhexine prophylaxis and its potential protective effects against respiratory viral infections.

Two heavy smokers who received prolonged bromhexine prophylaxis represent particularly relevant observations. One individual had been taking bromhexine continuously for approximately five years, whereas the second individual had used bromhexine prophylactically for approximately eight months annually over several consecutive years. Both individuals tolerated long-term bromhexine administration without reported adverse effects and, importantly, neither developed clinically apparent COVID-19 nor influenza during the observation period.

These observations are of interest because chronic smoking is associated with impaired respiratory epithelial function, increased susceptibility to respiratory infections, and a higher risk of severe outcomes following viral infections. Nevertheless, despite this increased vulnerability, both individuals remained free of clinically diagnosed COVID-19 and influenza while receiving prolonged bromhexine prophylaxis.

Although these cases cannot establish preventive efficacy in the absence of controlled comparison groups and systematic virological surveillance, they provide supportive real-world observations suggesting a favorable long-term tolerability profile and raising the possibility that sustained bromhexine prophylaxis may contribute to reduced clinical expression of respiratory viral infections.

Together with the other observations presented in this study, these findings support further prospective evaluation of bromhexine as a potential host-directed prophylactic strategy against respiratory viruses.

6. Conceptual Synthesis: Pharmacologically Attenuated Natural Immunization

Taken together, the observations described above support a unifying hypothesis regarding the potential role of bromhexine in modifying the host–virus interaction. Rather than acting as a conventional antiviral agent aimed at complete suppression of viral replication, bromhexine may function as a host-directed modulator by inhibiting TMPRSS2-dependent viral entry.

We propose the concept of pharmacologically attenuated natural immunization, whereby transient pharmacological inhibition of host-dependent viral entry pathways, such as TMPRSS2-mediated spike activation, reduces viral entry and disease severity while preserving sufficient antigen exposure to induce durable adaptive immunity.

In this model, the objective is not necessarily complete prevention of infection, but rather attenuation of viral entry and replication to permit controlled immune priming while minimizing clinically significant disease. By reducing viral burden below the threshold required for extensive tissue injury while maintaining sufficient antigen presentation, this approach may achieve a favorable balance between protection from disease and development of immune memory.

Conceptually, this proposed mechanism can be summarized as:

Bromhexine prophylaxis

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TMPRSS2 inhibition

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Reduced viral entry into respiratory epithelial cells

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Lower viral replication and reduced tissue injury

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Attenuated clinical expression of infection

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Preserved antigen presentation

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Activation of B-cell and T-cell responses

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Durable humoral and cellular immune memory

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Pharmacologically attenuated natural immunization

If validated in prospective studies, this concept may represent a distinct prophylactic paradigm compared with conventional antiviral approaches aimed primarily at complete suppression of viral replication. Rather than eliminating antigenic stimulation, partial inhibition of viral entry may allow controlled immune priming while substantially reducing the risk of clinically significant disease. Such an approach may be particularly relevant for respiratory viruses that depend on host proteases for efficient cellular entry.

7. Limitations and Future Perspectives

Nevertheless, these observations should be regarded as hypothesis-generating rather than conclusive. A causal relationship between bromhexine prophylaxis, attenuation of viral replication, and preservation of long-term adaptive immunity cannot be established from observational cases alone.

In addition, serological measurements cannot fully distinguish infection-induced immunity from residual vaccine-associated immune memory in individuals with previous vaccination. The exact timing of infection, viral load dynamics, and the contribution of cellular immunity cannot be determined from antibody measurements alone.

Future prospective studies incorporating documented exposure cohorts, serial viral load measurements, neutralizing antibody assays, and comprehensive cellular immune profiling will be required to determine whether TMPRSS2 inhibition by bromhexine can reproducibly achieve this proposed balance between attenuation of disease and preservation of durable adaptive immunity.

The observations presented here provide a rationale for investigating whether modulation of viral entry pathways can achieve protection through controlled attenuation rather than complete blockade of infection, introducing the possibility of a new host-directed approach to respiratory virus prophylaxis.

Although this review focuses on BRH as a TMPRSS2 inhibitor for prophylaxis, it also fits within a broader host-directed therapeutic strategy. Prevention of viral entry by bromhexine could be complemented by subsequent modulation of the excessive inflammatory response with colchicine through inhibition of the NLRP3 inflammasome (Figure 1). Together, these interventions target two distinct stages of disease pathogenesis—viral entry and hyperinflammation—and may represent a complementary approach for both COVID-19 and influenza [14,15,16]. Future studies should evaluate this integrated strategy in prospective clinical trials.