Why a single shot once offered a different path to immunity
In short: While most COVID-19 vaccines require multiple doses, the Johnson & Johnson vaccine was designed as a distinct technological approach. By using a viral vector, it aimed to provide protection in one go, but its unique mechanism also meant it eventually took a different trajectory than mRNA alternatives.
The Johnson & Johnson COVID-19 vaccine represents a specific category known as a viral vector vaccine. Unlike messenger RNA (mRNA) vaccines—which provide cells with instructions to produce the virus's spike protein—viral vector vaccines use a modified, harmless version of a different virus to deliver that same genetic information. This 'vector' essentially acts as a delivery vehicle, teaching the body to recognize and fight the COVID-19 S protein without causing an actual infection from either the vector or the target virus.
In contrast, other prominent technologies like mRNA vaccines (used by Pfizer-BioNTech and Moderna) work by sending instructions directly into muscle cells. Another distinct method is the protein subunit vaccine used by Novavax, which contains only the harmless S proteins themselves to stimulate an immune response. Because these mechanisms differ in how they alert the body to the risk of infection, their rollout and long-term usage have varied significantly across different regions.
The landscape of vaccination has shifted over time as protection fades and viruses evolve. While the Johnson & Johnson vaccine was a notable third approval for emergency use in the United States, it is no longer used in that country. Current guidance emphasizes that vaccines are essential tools to reduce the risk of severe symptoms, hospitalization, and death, particularly for high-risk groups such as those aged 75 or over, individuals with weakened immune systems, or residents of care homes.
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Sources
- Summary by Intellect · source: How The One-Shot COVID Vaccine Changes Things
- Also: Different types of COVID-19 vaccines: How they work
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