BEIJING -- A clinical trial in China has shown that an inhalable COVID-19 booster vaccine is safe and effective in adults.

Scientists led by Chen Wei with the Academy of Military Medical Sciences in China developed an aerosolized adenovirus type-5 vector-based COVID-19 vaccine (Ad5-nCoV) administered via oral inhalation.

Chen's group and the researchers from the Jiangsu Provincial Center for Disease Control and Prevention conducted a randomized, open-label, controlled trial to evaluate the vaccine's safety and immunogenicity as a booster jab in 420 Chinese adults who had previously received two doses of inactivated vaccine.

Eligible participants were randomly assigned between Sept. 14 and 16 last year to receive either a low dose or a high dose inhalable vaccine, or the vaccine identical to their previous two intramuscular shots, as a control group, according to the study published recently in the journal Lancet Respiratory Medicine.

The results showed that the low dose group had a concentration of serum neutralizing antibodies of 744.4 and the high dose group 714.1 14 days after the booster dose, significantly higher than that in the control group of 78.5.

The vaccine-induced neutralizing antibodies can bind to the coronavirus and prevent it from infecting cells.

Also, only 26 and 33 participants reported adverse reactions in the low dose group and the high dose group, respectively, within 14 days of the booster vaccination, significantly fewer than the 54 reported in the control group, according to the study.

The researchers concluded that a heterologous booster immunization with an aerosolized Ad5-nCoV in previously vaccinated adults is safe and highly immunogenic.

Now, they are planning a multicenter, randomized, double-blind, parallel-controlled efficacy trial of the inhaled vaccine as a booster dose following a primary series of immunization with Ad5-nCoV in Mexico.