Baja California is a State located in the north-western region of Mexico, which borders with the United States. This area is of particular importance in terms of epidemiological surveillance, given that the busiest port of entry to the U.S. is located in the Tijuana-San Diego region, which is also one of the busiest ports of entry in the world. During the first year of the COVID-19 pandemic, Baja California had the second mortality rate in Mexico due COVID-19. We conducted a cross-sectional study to know the prevalence of COVID-19 in the general population at the beginning of 2021, right after the second wave of COVID-19 cases in the State.

We carried out a population-based survey in the 3 largest cities of Baja California, Mexico: Mexicali, Tijuana, and Ensenada. The selection of households followed a probabilistic, 3 stage approach, with a total of 1,126 individuals included, which were representative of a population of 2.7 million after weighting. Real Time Polymerase Chain Reaction (RT-qPCR) was used to assess SARS-CoV-2 infection in nasopharyngeal swabs and IgG seroprevalence was evaluated in finger stick blood samples. We also assessed the level of knowledge, attitudes, and practices on COVID-19 prevention in the general population.

In general, Baja California had similar COVID-19 prevalence rate compared to other States in Mexico, and crossing the border with the U.S. was not associated to higher odds of infection. Ensenada had a higher prevalence of COVID-19 by RT-qPCR compared to Tijuana and Ensenada. The level of health literacy on COVID-19 shows important areas of opportunity, along with a considerable high rate of vaccine hesitancy.

Our findings on this important area in the Mexico-United States border show that the dynamics of COVID-19 are complex, and prevalence rates can significantly differ even within the same region. More work needs to be done in terms of health promotion in the general population of Baja California in order to achieve better health outcomes in future public health emergencies of international concern.

The survey followed a probabilistic approach for sampling at 3 levels; the basic geostatistical area, blocks, and households. The survey was administered by an interviewer and the data was entered in tablets with a previously installed app (KoboToolbox) for data entry. The results of sampling testing (RT-PCR and seroprevalence) was captured manually afterwards in the dataset generated by KoboToolbox.

We attach two read me files with detailed information about the questions in this survey and possible answers.