The hepatitis B (Hep B) vaccine is given at birth to protect babies from a virus that can silently infect them very early in life and cause lifelong liver disease, including cirrhosis and liver cancer. Giving the first dose within 24 hours of birth is a highly effective “safety net,” because a baby can be exposed during delivery or from infected caregivers whose infection is unknown.

What is hepatitis B?

  • Hepatitis B is a virus that attacks the liver and can lead to chronic infection, cirrhosis, liver failure, and liver cancer.
  • Over 240 million people worldwide live with chronic hepatitis B, many infected as infants or young children.
  • Babies infected at birth have a very high chance of becoming lifelong carriers, much higher than adults who get infected later.

Why give the vaccine at birth?

  • A baby can catch hepatitis B during childbirth if the mother has the virus, even if she appears completely healthy and does not know she is infected.
  • The birth dose given within 24 hours can prevent about 90% of these mother‑to‑child transmissions when used as recommended, dramatically lowering the risk of chronic infection.
  • Testing in pregnancy is very important, but it is not perfect: results can be delayed, missed, or occasionally fail to detect an infection, so the vaccine acts as a back‑up layer of protection.

Protection beyond the mother

  • Even if the mother tests negative, other household members or caregivers can carry hepatitis B without knowing it and could pass it to the baby through contact with small amounts of blood or body fluids (for example, through tiny cuts or shared items like toothbrushes or nail clippers).
  • The birth dose helps protect babies from these early “horizontal” exposures as they grow through infancy and early childhood.
  • In countries that introduced a universal birth dose for all newborns, infections in babies and young children dropped sharply, and future cases of liver cancer related to hepatitis B are expected to fall by thousands.

Why more than one dose?

  • The shot at birth is the first step; follow‑up doses at around 1–2 months and 6 months are needed to build strong, long‑lasting immunity.
  • The schedule is designed so the first dose protects during the most vulnerable period (birth and early weeks), while later doses “train” the immune system for durable protection into childhood and beyond.
  • Completing the full series is what turns that early safety net into long‑term protection against both early‑life and later‑life exposures.

Big‑picture impact and “latest” context

  • Universal newborn Hep B vaccination (a birth dose for every baby, not just high‑risk ones) was adopted after targeted approaches failed to stop infections, because many infected parents were never diagnosed.
  • Health agencies report that birth‑dose programs have sharply reduced chronic hepatitis B in children and moved countries closer to eliminating mother‑to‑child transmission.
  • Recent public discussions and fact‑checking efforts focus on correcting myths that the Hep B shot at birth is unnecessary “for an adult disease,” emphasizing that the main goal is preventing infections that start in infancy and silently cause disease decades later.

TL;DR: The Hep B vaccine is given at birth because that is when a baby is first at risk, infection at that age is the most dangerous and likely to become lifelong, and a dose in the first 24 hours is a proven, highly effective way to prevent chronic hepatitis B, cirrhosis, and liver cancer later in life.

Information gathered from public forums or data available on the internet and portrayed here.