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

Objective

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

Methods

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.

Results

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.

Conclusion

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.