The effectiveness of vaccines against COVID-19 in Mexico: A time series approach

To estimate the effectiveness of several vaccine brands-Pfizer, Astra, and Sinovac-against symptomatic COVID-19 without information on vaccination at the individual level.

We use data of mass vaccination programs-specifically, for sexagenarians and quinquagenarians-in three large municipalities of Mexico (Monterrey, Guadalupe, and San Nicolás) to conduct a two-step time series estimation procedure involving a synthetic control group. The data covers the period between the first week of March 2020 and the first week of October 2021.

Vaccine effectiveness is a concave function of time. At the peak, Pfizer reaches 92.6 % effectiveness, Astra 83.6 % and Sinovac 65.6 %. This occurs 9 to 12 weeks after the first shot.

The results indicate that the three vaccines protect against symptomatic COVID-19. Nevertheless, they offer different levels of protection. The results also suggest that VE-under a two-shot scheme-reaches its peak 9 to 16 weeks after the first shot. Moreover, there seems to be a trade-off between achieving higher efficiency by administering the 2nd shot earlier or extending the protection period by administering it later.