Mechanisms of Hepatitis B Virus Transmission
Hepatitis B virus (HBV) is primarily transmitted through percutaneous or mucosal exposure to infectious blood or body fluids containing blood. 1 This highly infectious virus can be transmitted even in the absence of visible blood and remains viable on environmental surfaces for at least 7 days.
Primary Routes of Transmission
Blood-borne Transmission
- Percutaneous exposure: Direct contact with infected blood through:
- Injection drug use (sharing needles/equipment)
- Needlestick injuries in healthcare settings
- Contaminated medical or dental instruments
- Unsafe injections
- Organ transplantation and dialysis procedures 1
Sexual Transmission
- HBV is efficiently transmitted through sexual contact among both heterosexuals and men who have sex with men (MSM)
- Risk factors for heterosexual transmission include:
- Unprotected sex with an infected partner
- Multiple sexual partners
- History of other sexually transmitted infections 1
- Risk factors for transmission among MSM include:
- Multiple sex partners
- History of other STIs
- Anal intercourse 1
Perinatal Transmission
- From infected mothers to newborns during childbirth
- Risk is highest (70-90%) when mothers are both HBsAg and HBeAg positive 1
- Risk is lower (<10%) when mothers are HBsAg positive but HBeAg negative 1
Household/Close Contact Transmission
- Sharing personal items (toothbrushes, razors)
- Contact with exudates from dermatologic lesions
- Contact with HBsAg-contaminated surfaces 1
- Long-term interpersonal contact with infected individuals 1
Infectious Body Fluids
HBV is concentrated most highly in blood, making percutaneous exposure the most efficient mode of transmission. However, other body fluids can also transmit the virus:
- Highly infectious: Blood (contains highest viral titers)
- Proven infectious: Semen, vaginal secretions, saliva 1
- Potentially infectious: Cerebrospinal fluid, synovial fluid, pleural fluid, peritoneal fluid, pericardial fluid, amniotic fluid 1
- Not efficient vehicles of transmission (unless they contain blood): Urine, feces, vomitus, nasopharyngeal washings, sputum, sweat 1
- Breast milk: HBsAg in breast milk is unlikely to lead to transmission; breastfeeding is not contraindicated 1
Viral Characteristics Enhancing Transmission
Several unique characteristics of HBV enhance its transmissibility:
- High infectivity: HBV is approximately 100 times more infectious than HIV and 10 times more infectious than HCV 1
- Environmental stability: Remains viable on environmental surfaces for at least 7 days 1
- Infectious without visible blood: Can be transmitted when no blood is visible 1
- High viral titers: HBsAg-positive persons with HBeAg have extremely high viral loads (typically 10^7-10^9 virions/mL) 1
- Low infectious dose: HBV at concentrations of just 10^2-10^3 virions/mL can cause transmission 1
Special Populations at Risk
- Injection drug users: High risk due to sharing of contaminated needles and equipment
- Sexual partners of infected individuals: Risk increases with multiple partners and unprotected sex
- Infants born to infected mothers: High risk of perinatal transmission without prophylaxis
- Household contacts of infected persons: Risk from sharing personal items and prolonged contact
- Healthcare workers: Risk from needlestick injuries and exposure to infected blood
Prevention Considerations
Understanding these transmission mechanisms has led to effective prevention strategies:
- Universal vaccination of infants, children, and high-risk adults
- Screening of pregnant women and prophylaxis for infants born to infected mothers
- Safe injection practices and blood screening
- Safe sex practices and condom use
- Avoiding sharing of personal items with infected individuals
The complex interplay between the virus's high infectivity, environmental stability, and multiple transmission routes explains why hepatitis B remains a significant global health challenge despite the availability of an effective vaccine.