How big is Hepatitis B Virus? | Scale of the Universe

The hepatitis B virus is the virus that causes hepatitis B, an infection of the liver. A complete, infectious particle of the virus is a sphere about 42 nanometers across, and a nanometer is a billionth of a meter. That width is measured across the whole particle, including its outer envelope.

Size

Complete hepatitis B particles are called Dane particles, after the lead author of a 1970 paper that described them in electron microscope images. Reviews of the virus give their diameter as 42 nanometers. The International Committee on Taxonomy of Viruses (ICTV), which classifies viruses, gives a range of 40–45 nanometers.

Each particle has two layers. The outer one is an envelope made of fatty molecules called lipids, studded with three kinds of surface protein. Inside it is a protein shell called the nucleocapsid, 32–36 nanometers across. The nucleocapsid holds the virus's genetic material: a loop of DNA about 3,200 bases long, only partly double-stranded.

Size comparisons

A hepatitis B virus is about 2.5 times as wide as a porcine circovirus, a virus of pigs about 17 nanometers across. By volume, it is about 15 times as large.
In 1873, Ernst Abbe worked out that an ordinary light microscope cannot resolve details smaller than about 200 nanometers. A hepatitis B virus is about one-fifth of that limit, too small for such a microscope to show its shape.
About 1,900–2,400 hepatitis B viruses lined up side by side would span a human hair, which is 80,000–100,000 nanometers wide.
Enlarged a million times, a hepatitis B virus would be 4.2 centimeters (1.7 inches) across, and a human hair would be 80–100 meters (260–330 feet) wide.

Empty particles

Infected liver cells also release particles that contain no nucleocapsid and no DNA, so they cannot cause infection. They are made of surface proteins set in a lipid membrane taken from the cell. Some are spheres 16–25 nanometers across, and others are filaments, or threads, about 20 nanometers wide and of varying length.

These empty particles far outnumber complete viruses: a 2025 review puts them at 1,000 to 100,000 times as many. They soak up the antibodies the immune system makes against the surface proteins, drawing antibodies away from the infectious particles. Their surface proteins were first identified as the "Australia antigen".

Infection and vaccine

Hepatitis B spreads through blood and other body fluids, and most commonly from mother to child at birth. The only known natural hosts of the virus are humans and apes. Most people have no symptoms when first infected. In some, though, the infection becomes chronic, or long-lasting, and can lead to serious liver disease, including scarring of the liver, called cirrhosis, and liver cancer.

Age matters. About 95% of infants and young children who are infected develop chronic hepatitis, compared with fewer than 5% of infected adults. The World Health Organization estimates that 240 million people were living with chronic hepatitis B in 2024, and that the disease caused about 1.1 million deaths that year. A vaccine, usually given soon after birth, offers nearly 100% protection.