🩹 Hepatitis B Post-Exposure Prophylaxis Simulator

Checking the Exposed Person’s Vaccination History and Antibody Titer

Post-exposure prophylaxis (PEP) decision-making for hepatitis B begins not with the exposure itself, but with a question about the past: has this person already been vaccinated, and if so, is there documented proof their immune system responded? This single fact — vaccination history plus, when available, a quantitative anti-HBs titer — determines almost the entire subsequent management pathway, before the source patient is even considered.

• ≥10 mIU/mL: Protective anti-HBs threshold (measured 1–2 months post-series)
• 3 doses: Standard vaccine series (0, 1, and 6 month schedule)
• ≥30 years: Immune memory durability (even after titer wanes below threshold)
• 0%: Documented responders needing PEP (no HBIG or extra doses required)

Why vaccination history is checked first

Hepatitis B post-exposure management is fundamentally different from most other PEP protocols because it depends heavily on pre-existing immunity rather than solely on the character of the exposure. A person who completed the 3-dose hepatitis B vaccine series and mounted an adequate antibody response already carries durable, long-lived protection — memory B cells and long-lived plasma cells that can be reactivated by a new antigen exposure within days, well inside the incubation period of the virus.

Because of this, the very first step of any hepatitis B PEP evaluation is a structured history: Has the exposed person ever received the hepatitis B vaccine series? Was it completed (all 3 doses)? Was post-vaccination serologic testing ever performed, and if so, what was the result?

This reframes the entire encounter: instead of starting from "what happened during the exposure," clinicians start from "what protection does this person already have." The exposure details (needlestick, mucosal splash, sexual contact) matter for risk stratification later, but the immune status check comes first because it can make the rest of the workup unnecessary.

Interpreting a known anti-HBs titer

When a documented quantitative anti-HBs (antibody to hepatitis B surface antigen) result exists, interpretation is straightforward: a titer ≥10 mIU/mL measured 1–2 months after completing the vaccine series defines an "adequate response," and this person is considered immune for life in essentially all practical clinical scenarios, even if a repeat titer years later comes back lower or undetectable — because anamnestic (memory) response occurs rapidly upon re-exposure.

If no post-vaccination titer was ever checked, the person is classified as having "unknown response," which for PEP purposes is treated similarly to being unprotected until proven otherwise, because roughly 5–10% of healthy vaccine recipients are true non-responders (failing to reach protective titers even after a full series, sometimes even after a second full series).

Rapid point-of-care or same-day laboratory anti-HBs testing, when available, can resolve this uncertainty quickly and should be pursued when it will not meaningfully delay other components of prophylaxis.

Weighing the Source Patient’s HBsAg Status Without Delaying Care

Once the exposed person’s own immune status is established, attention turns to the source of the exposure. Knowing whether the source is HBsAg-positive, HBsAg-negative, or of unknown status sharpens the risk assessment considerably — but hepatitis B PEP is time-sensitive, and in real practice, decisions are frequently made pragmatically without waiting for source testing to return.

• up to 30%: HBV transmission risk (needlestick, HBeAg+) (vs. <0.3% for HIV in similar exposures)
• ~296 M: People living with chronic HBV worldwide (WHO global estimate)
• ~30 min: Rapid HBsAg test turnaround (when point-of-care testing is available)
• ≤24 h ideal, ≤7 days usable: HBIG effectiveness window (efficacy declines with delay)

What source testing changes — and what it doesn’t

If the source patient is known and consents to testing, an HBsAg result clarifies the exposure risk: a positive result confirms active infection and elevated transmission risk; a negative result essentially removes the need for HBIG (though the vaccine series is still advisable for an unprotected exposed person, since any blood exposure carries some baseline value in completing immunization). An unknown or untestable source is treated as at least moderate-to-high risk, especially in settings (emergency departments, needlestick from a discarded needle, sexual assault) where the epidemiological prevalence of HBV in the source population is not low.

Crucially, source testing refines but does not gate the decision to protect the exposed person. Because HBIG efficacy is highest when given within 24 hours and drops off over the following days, clinicians are taught not to wait for source lab results if the exposed person’s immune status already indicates a need for prophylaxis — the vaccine series (and HBIG, if indicated on other grounds) is started immediately, with the source result incorporated afterward if it changes the plan.

Pragmatic proceeding: acting before all data is in

This is one of the more counter-intuitive aspects of hepatitis B PEP for learners: the workflow is explicitly designed to avoid "analysis paralysis." Occupational health and emergency protocols consistently emphasize that PEP should not be delayed pending source testing results, particularly when the exposed person is unvaccinated or a non-responder, because the marginal cost of proceeding with HBIG/vaccine in a case that turns out to be low-risk is small, while the cost of delaying prophylaxis in a true high-risk exposure (chronic active hepatitis B infection, cirrhosis, hepatocellular carcinoma decades later) is severe and irreversible.

In practice, most exposure-management algorithms therefore run two decisions in parallel: (1) immediately protect the exposed person based on their own vaccination status, and (2) test the source as fast as feasible to fine-tune — rarely to reverse — that decision.

The Reassuring Pathway: Existing Immunity Needs No Reinforcement

For the subset of exposed people who can show a documented adequate antibody response to a prior complete vaccine series, hepatitis B PEP resolves quickly and reassuringly: no HBIG, no additional vaccine doses, and no special monitoring beyond routine follow-up. Their immune system has already demonstrated it can mount and sustain a protective response.

• 0 doses: HBIG required for documented responders (passive immunization unnecessary)
• 0 doses: Extra vaccine doses required (series is already complete and effective)
• exceedingly low: Breakthrough infection risk after adequate response (protection is durable long-term)
• none: Additional clinical action needed (routine reassurance only)

Why documented response is treated as definitive

The immunologic rationale for closing the case at this point is well established: once a person has generated anti-HBs ≥10 mIU/mL after a complete 3-dose series, they retain immune memory capable of a rapid anamnestic antibody surge upon re-exposure to hepatitis B surface antigen — typically within days, well before the virus could establish a productive infection during its multi-week incubation period. This is true even in individuals whose measured circulating antibody titer has since declined below the 10 mIU/mL threshold on a later blood draw; memory B cells persist long after circulating antibody levels fall.

Because of this durability, essentially every major hepatitis B PEP guideline treats "documented adequate responder" as a stopping point: no HBIG, because passive antibody would add nothing to an immune system already capable of its own rapid response, and no extra vaccine dosing, because the person’s prior series already achieved its immunologic goal.

What still happens at the visit

Even though no pharmacologic intervention is needed, the exposure encounter is not simply dismissed. Standard practice still includes: documenting the exposure event itself (for occupational health records and, where relevant, workers’ compensation purposes), providing reassurance and education to the exposed person about why no treatment is required, and completing any other exposure-specific evaluation unrelated to hepatitis B (for example, HIV or hepatitis C risk assessment, which follow entirely separate decision trees and are not resolved by hepatitis B immunity).

This pathway illustrates a broader principle in PEP algorithms: the fastest, safest outcome is often "no action needed," and recognizing when prior immunization has already done the job avoids unnecessary injections, cost, and patient anxiety.

Combining Passive and Active Immunization for Unprotected Individuals

For people who are unvaccinated, or who completed a series but are documented non-responders, hepatitis B PEP relies on a two-pronged strategy: hepatitis B immune globulin (HBIG) for immediate, short-term passive protection, paired with initiating or completing the vaccine series to build durable, active, long-term immunity. Neither component alone is sufficient for this group.

• 0.06 mL/kg: HBIG standard dose (intramuscular injection)
• 3 doses: Vaccine doses for unvaccinated (0, 1, and 6 month schedule)
• 2nd series or 2× HBIG: True non-responder management (1 month apart, per protocol)
• within 24 h: Ideal HBIG timing (administer at a different site than vaccine)

HBIG: immediate passive protection

Hepatitis B immune globulin is a pooled, hyperimmune plasma-derived antibody preparation containing high titers of anti-HBs, collected from donors with robust immunity to hepatitis B. When injected intramuscularly (typically 0.06 mL/kg), it delivers ready-made antibodies directly into circulation, neutralizing circulating hepatitis B virus particles and surface antigen before they can establish infection in hepatocytes.

This passive protection is fast — effective within hours — but temporary, with a half-life of roughly 3–4 weeks, meaning HBIG alone cannot provide durable protection. It buys time while the person’s own immune system, primed by the concurrently started vaccine series, ramps up an active antibody response of its own. Because HBIG efficacy is time-sensitive, it is most effective within 24 hours of exposure and generally still considered useful up to about 7 days afterward, but effectiveness declines the longer administration is delayed.

Vaccine series: durable active immunity

The hepatitis B vaccine is a recombinant subunit vaccine containing purified hepatitis B surface antigen (HBsAg), produced in yeast cells and formulated with an adjuvant. Administered intramuscularly (deliberately at a different injection site than HBIG, to avoid antibody neutralization interfering with the vaccine antigen), it stimulates the recipient’s own B cells to generate anti-HBs antibodies and, critically, establish long-lived immune memory.

For unvaccinated individuals, the standard 3-dose series (at 0, 1, and 6 months) is initiated immediately after the exposure, ideally alongside the first dose of HBIG. For documented non-responders to a prior series, guidance typically recommends either a full second 3-dose vaccine series, or two doses of HBIG spaced one month apart if the person is a documented double non-responder — since a third vaccine attempt has a diminishing chance of success in that specific population.

Why both are needed together

HBIG and vaccine address two different time windows of vulnerability. HBIG covers the immediate days-to-weeks period after exposure, before an active immune response could plausibly develop from a first vaccine dose — without it, an unprotected person exposed to a high-risk source has no defense during this critical early window. The vaccine series, in turn, covers everything beyond that window, for the rest of the person’s life, but takes weeks to months to fully mature into a protective, memory-based response.

Giving only HBIG would leave the person unprotected once the passive antibodies wane in about a month. Giving only the vaccine would leave a dangerous gap during the exposure’s incubation period, when the virus could establish infection before the vaccine has had time to generate a response. Combining both closes this gap completely for people who otherwise have no existing immunity to fall back on.

HBIG plus vaccine, given correctly and promptly, is more than 90% effective at preventing hepatitis B transmission after a high-risk exposure — among the most effective post-exposure interventions in infectious disease medicine.

The Pragmatic Pathway When Status Cannot Be Quickly Confirmed

Real-world exposures do not always come with clean documentation. When the exposed person’s vaccination history or antibody response is unknown, and cannot be resolved through quick testing, hepatitis B PEP protocols default to a pragmatic, risk-averse strategy: treat the person as if unprotected, start the vaccine series without delay, and add HBIG based on the assessed risk contributed by the source patient.

• unprotected: Default assumption when status unknown (safest working assumption)
• start series now: Action taken while status is clarified (first vaccine dose given immediately)
• source high-risk or unknown: HBIG added when (cannot be quickly ruled out)
• prior immunity later confirmed: Plan revised if (remaining doses can be stopped)

Defaulting to the safer assumption

When rapid anti-HBs or vaccination-record verification is not available at the point of care — a common scenario in emergency departments, correctional facilities, or when records from another country or provider cannot be retrieved quickly — clinicians are guided to act rather than wait. The working assumption becomes that the exposed person is unprotected, mirroring the unvaccinated/non-responder pathway: begin the vaccine series immediately, since there is essentially no downside to vaccinating someone who turns out to already be immune, beyond one extra, low-risk injection.

This "assume the worst, act now, refine later" logic mirrors similar pragmatic defaults used across post-exposure prophylaxis medicine generally (for example, in HIV PEP when a source’s status is unknown) — the cost of unnecessary treatment is far smaller than the cost of a missed window for effective prevention.

Layering in HBIG based on source risk

Whether HBIG is added on top of the newly started vaccine series depends on what can be learned about the source, even under time pressure: a source known or strongly suspected to be HBsAg-positive (or belonging to a high-prevalence group, or simply unidentifiable/untestable in a high-risk setting such as a shared-needle exposure) tips the decision toward adding HBIG, following the same reasoning as the unvaccinated/non-responder pathway. A source that can be quickly confirmed HBsAg-negative allows HBIG to be reasonably withheld, since the marginal exposure risk is low even for an unprotected recipient.

This stage effectively merges the logic of Stages 1, 2, and 4: because the exposed person’s own status is the unresolved variable, the source’s status becomes the primary lever available for fine-tuning how aggressively to intervene.

If later testing or records confirm the exposed person actually had documented immunity all along, remaining scheduled vaccine doses can simply be stopped — the pragmatic pathway is designed to be safely reversible, not a lasting overcommitment.

Why speed matters more than certainty here

The unifying theme across the uncertain-status pathway is that hepatitis B PEP effectiveness is strongly time-dependent, particularly for HBIG, whose benefit declines the longer administration is delayed. Waiting several days to definitively resolve a patient’s vaccination history — tracking down old records, repeating serology, contacting prior providers — can consume exactly the window in which prophylaxis is most effective.

By proceeding pragmatically now and adjusting later if better information arrives, this pathway prioritizes not missing the treatment window over achieving diagnostic certainty before acting, which is precisely the design intent of post-exposure prophylaxis protocols in general: prevention decisions made under uncertainty, on a clock.