The six-month follow-up medical observation for China's first batch of COVID-19 vaccine volunteers ends on the weekend, as the volunteers complete the final blood sample collection to see antibody levels.

The research team is planning to give volunteers a second dose on a voluntary basis to boost their immunity against the novel coronavirus, the Global Times learned from the research team. Other Chinese vaccine candidates entering the late-stage trials all require at least two doses to maximize immune response.

China's earliest group of 108 volunteers started to get their injections on March 17 in Wuhan for Phase I clinical trials of the Recombinant Novel Coronavirus Vaccine (Ad5-nCoV), co-developed by Tianjin-based biotechnology company CanSino Biologics Inc and military infectious diseases expert Chen Wei.

"We took about 35 millimeters of blood samples this morning. The second dose may be given in the following two days. I did not experience any adverse reactions, no colds, no fevers, over the past six months," Zhu Aobing, 28, one of the first group of volunteers, told the Global Times on Saturday.

The vaccine candidate is originally designed for single-dose injection, estimated to stay effective for two years and should be stored at a temperature of 2-8 degrees Celsius.

But now the research team intends to extend it to two doses in order to bring better protection to volunteers, a researcher participating in the clinical trials in Wuhan told the Global Times on Saturday. If an extra dose of the vaccine is given, the team will start a new six-month period of medical follow-up observations.

No severe adverse effects from Ad5-nCoV have been discovered so far, Zhu Tao, CanSino's chief scientific officer, said over the last weekend at a conference.

The vaccine's effectiveness has been called into question due to tests showing low antibodies in previously published Phase 1 and 2 trial data. However, Zhu said that the neutralizing antibody of the vaccine may have been seriously underestimated, and that a single dose of it can achieve the equivalent immunity effect of two doses of inactivated vaccine. He also stressed that different testing methods can seriously affect antibody testing data.

The data show that the Ad5-nCoV vaccine is no less effective than the same-typed Ad26 vaccine already approved in Russia, Zhu added.

Other three Chinese-developed vaccine candidates also take two doses of injection including two inactivated vaccines developed by China National Pharmaceutical Group Sinopharm in Beijing, and another inactivated one developed by Sinovac Biotech Ltd.

A 28-day interval between two doses is a normal scheme, while a 14-day interval is acceptable under emergency situations, Yin Weidong, CEO of Sinovac Biotech Ltd, told the media.

Whether a third dose is necessary is still under observation now, but the team has already designed a third dose in its Phase II clinical trials and will report the results soon, said Yin. He suggested not to worry too much about a decrease in antibody level over time because the extra shot can consolidate the immune effects.

Chinese vaccine companies have signed cooperation agreements with institutions in several countries to promote final-stage clinical trials, Wu Yuanbin, director-general of science and technology for social development with the Ministry of Science and Technology, said at a briefing held by the State Council Information Office of China on Friday.

Sinopharm's two inactivated vaccines have entered Phase III clinical trials in the Middle East, with more than 35,000 people vaccinated and no serious adverse reactions. Sinovac Ltd is smoothly running its Phase III clinical trials for its inactivated vaccine in South American and Southeast Asian countries. Volunteers for the late-stage trials of the Ad5-nCoV are also starting injections in a few countries in Eurasia, Wu briefed.