KUALA LUMPUR, Nov 12 — A real-world study in Chile found that administering Pfizer or AstraZeneca as a third dose for recipients double vaccinated with Sinovac’s Covid-19 vaccine was more effective than a third Sinovac jab.
According to the “Covid-19 vaccine effectiveness assessment in Chile” study released by the Chilean government at a World Health Organization (WHO) consultation on October 25, efficacy in preventing SARS-CoV-2 infection — among individuals double vaccinated with Sinovac — rose to about 93 per cent for a third dose with Pfizer, 91 per cent for AstraZeneca, and 71 per cent for Sinovac.
The 71 per cent efficacy rate for a third Sinovac jab, 14 days after the third shot, was lower than 81 per cent efficacy with two Pfizer-BioNTech doses, 14 days after the second dose. Two Sinovac doses only yielded 50 per cent effectiveness against coronavirus infection, 14 days after the second shot.
In terms of symptomatic Covid-19 disease, efficacy in people fully vaccinated with Sinovac increased to about 95 per cent for a third shot with Pfizer, 94 per cent for AstraZeneca-Oxford, and 74 per cent for a third Sinovac dose.
Again, three Sinovac doses was less effective in preventing Covid-19 disease at 74 per cent than two Pfizer doses with 84 per cent efficacy. Sinovac was only 54 per cent effective in preventing symptomatic Covid-19 with two doses, 14 days after the second shot.
Heterologous booster vaccination with AstraZeneca or Pfizer for Sinovac recipients was more effective against hospital admissions at 97 per cent and 91 per cent respectively, compared to 81 per cent for a third Sinovac jab.
Three Sinovac doses were less effective in preventing hospitalisation from Covid-19 at 81 per cent than two Pfizer doses (94 per cent) and two AstraZeneca doses (87 per cent).
In preventing intensive care unit (ICU) admission from Covid-19, effectiveness rose to 99 per cent for a third shot with AstraZeneca for those double vaccinated with Sinovac, 93 per cent for a third dose with Pfizer, and 85 per cent for an additional Sinovac jab.
Three Sinovac doses were less effective against ICU admission at 85 per cent than two Pfizer doses (97 per cent) and two AstraZeneca doses (95 per cent).
Strangely, efficacy against hospitalisation and ICU admission declined for a third Sinovac dose compared to two Sinovac doses, from 83 per cent to 81 per cent (hospital admissions) and from 87 per cent to 85 per cent (ICU admissions).
CanSino, meant as a single-dose vaccine, showed the poorest efficacy against Covid-19 infection and symptomatic disease compared to other vaccines. Twenty-eight days after the second CanSino dose, the vaccine was 50 per cent effective against coronavirus infection, and 52 per cent effective against symptomatic Covid-19 disease.
In terms of preventing hospitalisation and ICU admission, CanSino was 84 per cent and 95 per cent effective respectively after the second dose, slightly higher than two Sinovac doses at 83 per cent and 87 per cent respectively.
“We provide observational evidence to support that heterologous and homologous booster shots were able to significantly increase the effectiveness of primary immunization among people vaccinated with inactivated vaccines,” wrote study authors Dr Rafael Araos and Alejandro Jara, together with Chile’s Ministry of Health.
The Chile real-world study, which looked at 11 million people with Covid-19, including 2.9 million vaccinated with three doses, did not look at efficacy in preventing death with an additional vaccine dose as no deaths were reported in those who received boosters.