The second dose of Johnson & Johnson's COVID-19 vaccine generates a strong immune response against the deadly virus among people previously vaccinated with its single-shot vaccine, the company said.

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New interim data demonstrate that a booster dose of the J&J's COVID-19 vaccine generated a rapid and robust increase in spike-binding antibodies. People who received booster doses showed a ninefold increase in antibody levels compared with one month after the first dose, the US drugmaker said in a statement on 25 August.

The company said researchers observed significant increases in antibody responses in participants between ages 18 and 55 and in those 65 and older who received a lower booster dose. However, the company didn't specify exactly when or how many subjects received the second dose. Further, J&J stated that the study has been submitted on pre-print medRxiv, meaning that the data haven't been peer-reviewed or published in a scientific journal.

The company had, in July, reported in interim Phase 1/2a data that a single shot of the COVID-19 vaccine generates strong and robust immune responses that are durable and persistent through eight months. In the "new data, we also see that a booster dose of the Johnson & Johnson COVID-19 vaccine further increases antibody responses among study participants who had previously received our vaccine", said Mathai Mammen, Global Head, Janssen Research and Development, J&J, in the statement.

"We look forward to discussing with public health officials a potential strategy for our Johnson & Johnson COVID-19 vaccine, boosting eight months or longer after the primary single-dose vaccination," he added. The company is engaging with the US Food and Drug Administration (FDA), US Centers for Disease Control and Prevention (CDC), European Medicines Agency (EMA) and other health authorities regarding boosting the J&J COVID-19 vaccine.

The Johnson & Johnson COVID-19 single-dose vaccine is compatible with standard vaccine storage and distribution channels with ease of delivery to remote areas. The vaccine is estimated to remain stable for two years at minus 4 degrees Fahrenheit (minus 20 degrees Celsius), and a maximum of 4.5 months at routine refrigeration temperatures of 36-46 degrees Fahrenheit (2-8 degrees Celsius). According to the CDC data, 14 million J&J vaccines have been administered in the US -- the least used vaccine in the country by a significant margin.

The others, Moderna and Pfizer, have been respectively administered more than 140 million and 205 million times, although two doses are needed for full immunisation, CDC data showed.