Photo: Courtesy of Sinovac

A third dose of Sinovac's inactivated vaccine increases the potency, scale, and duration of anamnestic responses against SARS-CoV-2, according to the latest lab study data published on Saturday in a pre-print paper on medRvix .

Also, a full course immunization with Sinovac's inactivated vaccines could effectively protect against severe illness caused by the Delta variant, according to research results of the real-world study on patients infected with the Delta variant in the recent outbreak in East China's Jiangsu Province.

Administration of a third dose of an inactivated virus vaccine can boost the immune response and raise a better neutralization breadth and long-lasting humoral response in warding off COVID-19, the data suggest after researchers analyzed 171 complex structures of SARS-CoV-2 neutralizing antibodies.

Current research shows that the effectiveness of the vaccines has started to decline as months pass after vaccination due to fading immunity, the paper said. "Our results demonstrate that a third dose booster of inactivated vaccine can elicit an expeditious, robust and long-lasting recall humoral response which continues to evolve with ongoing accumulation of somatic mutations, emergence of new clones and increasing affinities of antibodies to antigens, conferring enhanced neutralizing potency and breadth."

Collectively, the findings rationalize the use of three-dose vaccination regimens, the paper noted.

Furthermore, another paper also published on medRvix suggests that the risk of progression to severe illness had substantially decreased in fully vaccinated patients infected with the Delta variant. 

The Delta variant, which was first identified in India, is more transmittable than other lineages of SARS-CoV-2 and now is becoming the major strain causing the COVID-19 pandemic.

This is the largest real-world study confirming the effectiveness of inactive COVID-19 vaccines against severe illness in patients infected with the Delta variant in Jiangsu, China, according to the paper published on Thursday.

Underlying comorbidities and old age are risk factors for severe illness in SARS-CoV-2 infected patients. However, it was found that severe illness did not occur in fully vaccinated patients without underlying medical conditions, showing a 100 percent protection.

Both of the two fully vaccinated patients who developed severe illness had underlying diseases. And 100-percent protection was also found in elderly patients that had been fully vaccinated, the paper said.

"The protective effect is affected by underlying medical conditions. Partial vaccination does not offer clinically meaningful protection against severe illness. Our study highlights the importance of continuing efforts on a full course of vaccination," the research concluded.

Remarkably, severe illness was 100 percent prevented in fully vaccinated women , while fully vaccinated men had only 81 percent reduced risk. Whether sex disparities exist in COVID-19 vaccine efficacy needs to be further explored.