SARS-CoV-2 IgG Antibodies Seroprevalence and Sera Neutralizing Activity in MEXICO: A National Cross-Sectional Study during 2020

Detection of IgG antibodies against the receptor binding domain of the spike protein and nucleocapsid of SARS-CoV-2 at university students from Southern Mexico: a cross-sectional study

BACKGROUND

Natural infection and vaccination against SARS-CoV-2 is associated with the development of immunity against the structural proteins of the virus. Specifically, the two most immunogenic are the S (spike) and N (nucleocapsid) proteins. Seroprevalence studies performed in university students provide information to estimate the number of infected patients (symptomatic or asymptomatic) and generate knowledge about the viral spread, vaccine efficacy, and epidemiological control. Which, the aim of this study was to evaluate IgG antibodies against the S and N proteins of SARS-CoV-2 at university students from Southern Mexico.

METHODS

A total of 1418 serum samples were collected from eighteen work centers of the Autonomous University of Guerrero. Antibodies were detected by Indirect ELISA using as antigen peptides derived from the S and N proteins.

RESULTS

We reported a total seroprevalence of 39.9% anti-S/N (positive to both antigens), 14.1% anti-S and 0.5% anti-N. The highest seroprevalence was reported in the work centers from Costa Grande, Acapulco and Centro. Seroprevalence was associated with age, COVID-19, contact with infected patients, and vaccination.

CONCLUSION

University students could play an essential role in disseminating SARS-CoV-2. We reported a seroprevalence of 54.5% against the S and N proteins, which could be due to the high population rate and cultural resistance to safety measures against COVID-19 in the different regions of the state.

Association of Obesity with SARS-CoV-2 and Its Relationship with the Humoral Response Prior to Vaccination in the State of Mexico: A Cross-Sectional Study

Obesity is associated with an increased risk of contracting infections. This study aimed to estimate the risk of COVID-19 infection associated with obesity and to assess its role in the specific antibody response against SARS-CoV-2 in 2021. This study included 980 participants from the State of Mexico who participated in a serological survey where they were tested for SARS-CoV-2 IgG anti-S1/S2 and anti-RBD antibodies and asked for height, weight, and previous infection data via a questionnaire. Of the cohort of 980 participants, 451 (46.02%) were seropositive at the time of recruitment (45.2% symptomatic and 54.8% asymptomatic). The risk of SARS-CoV-2 infection with obesity was 2.18 (95% CI: 1.51-3.16), 2.58 (95% CI: 1.63-4.09), and 1.88 (95% CI: 1.18-2.98) for seropositive, asymptomatic, and symptomatic individuals, respectively, compared to those with normal weight. Anti-S1/S2 and anti-RBD IgG antibodies tended to be higher in overweight and obese participants in the seropositive group and stratified by different obesity classes. Additionally, there was a positive correlation between anti-S1/S2 and anti-RBD IgG antibodies and BMI in both men and women in the seropositive group. Obesity is an independent risk factor for SARS-CoV-2 infection when adjusted for confounding variables; however, the relationship between BMI and anti-S1/S2 and anti-RBD IgG antibody levels differed markedly in the presence or absence of symptoms.

Comparing the evolutionary dynamics of predominant SARS-CoV-2 virus lineages co-circulating in Mexico

Over 200 different SARS-CoV-2 lineages have been observed in Mexico by November 2021. To investigate lineage replacement dynamics, we applied a phylodynamic approach and explored the evolutionary trajectories of five dominant lineages that circulated during the first year of local transmission. For most lineages, peaks in sampling frequencies coincided with different epidemiological waves of infection in Mexico. Lineages B.1.1.222 and B.1.1.519 exhibited similar dynamics, constituting clades that likely originated in Mexico and persisted for >12 months. Lineages B.1.1.7, P.1 and B.1.617.2 also displayed similar dynamics, characterized by multiple introduction events leading to a few successful extended local transmission chains that persisted for several months. For the largest B.1.617.2 clades, we further explored viral lineage movements across Mexico. Many clades were located within the south region of the country, suggesting that this area played a key role in the spread of SARS-CoV-2 in Mexico.

Association of Toxoplasmosis and COVID-19 in a MexicanPopulation

SARS-CoV-2 is the causal agent of COVID-19; the first report of SARS-CoV-2 infection was in December 2019 in Wuhan, China. This virus has since caused the largest pandemic in history, and the number of deaths and infections has been significant. Nevertheless, the development of vaccines has helped to reduce both deaths and infections. Comorbidities such as diabetes, hypertension, heart and lung diseases, and obesity have been identified as additional risk factors for infection and the progression of COVID-19. Additionally, latent toxoplasmosis has been reported to be a risk factor for acquiring COVID-19 in some studies, but other studies have suggested a negative association between these two infections. Furthermore, in patients after vaccination or with COVID-19 and coinfection, an increase in the lethality and mortality of toxoplasmosis has been observed. Therefore, the objective of the current study is to determine the association of toxoplasmosis with COVID-19 in patients diagnosed with COVID-19. Serum samples from 384 patients previously diagnosed with COVID-19 using IgG antibodies against the S1/S2 antigens of SARS-CoV-2 were collected. Subsequently, anti-Toxoplasma IgG and IgM antibodies were analyzed with ELISA. Statistical analysis was performed using SPSS Version 20.0 frequencies, percentages, 2 × 2 tables, and the Pearson correlation coefficient. IgG and IgM anti-Toxoplasma antibodies were positive in 105/384 (27.34%) and (26/191) 13.6% of patients, respectively. The positivity for both infections was higher in patients aged >40 years old. Subjects who were overweight or obese were mainly positive for both IgG antibodies against S1/S2 SARS-CoV-2 and Toxoplasma antibodies. In conclusion, the coinfection rate was 21.7%. The prevalence of S1/S2 SARS-CoV-2 was 308/384 (80.2%), and the percentage of Toxoplasma antibodies was 27.34%.

Special Issue “SARS-CoV-2: Epidemiology and Pathogenesis”: Editorial

Since its emergence in 2019 in Wuhan City, Hubei Province, China, SARS-CoV-2 has spread across hundreds of countries and all continents [...]

Longevity and Neutralizing Capacity of IgG Antibodies against SARS-CoV-2 Generated by the Application of BNT162b2, AZD1222, Convidecia, Sputnik V, and CoronaVac Vaccines: a Cohort Study in the Mexican Population

The WHO has approved the use of several vaccines during the COVID-19 pandemic; experience over the last 2 years has indicated that dose demand can only be covered using more than one design. Therefore, having scientific evidence of the performance of the different vaccines applied in a country is highly relevant. In Mexico, 5 vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) were used, allowing a cohort study to analyze the generation of anti-S1/S2 IgG antibodies and anti-RBD antibodies with neutralizing activity at 0, 21, 90, and 180 days after vaccination. Five groups of participants were formed on the basis of the type of vaccine received and were divided on the basis of whether they previously had or did not have COVID-19. After completing the vaccination schedule, the seroprevalence was 95.5, 97.5, 81.0, 95.2, and 90.0% (BNT162b2, AZD1222, Convidecia, Sputnik V, and CoronaVac, respectively). Among the participants without COVID-19 prior to vaccination, the largest amount of antibodies in the 90-day period was observed in the BNT162b2 group, and the amount of antibodies in the Sputnik V group decreased the least over time. Even though the percentages of seroconversion obtained in this study were lower than those currently reported in other parts of the world, the tested vaccines are able, in most cases, to induce a good production of IgG antibodies anti-S1/S2 and neutralizing capacity. The fact that there are people who have not produced antibodies during the study leaves open some questions that must be investigated to avoid the appearance of serious cases of COVID-19. IMPORTANCE Since the start of the vaccination programs against COVID-19 in 2020, it was evident that due to global shortages, the demand for the dose required in Mexico could only be covered by acquiring different vaccines. Therefore, determining the effectiveness of these and the longevity of acquired immunity is extremely important in a scenario where SARS-CoV-2 circulation becomes endemic and booster doses are required periodically. Our data reveal significant differences both in the generation of antibodies as well as in their longevity for the vaccines applied in the country but suggest that, in general, the Mexican population can reach a high capacity to neutralize the virus, therefore, regarding less the variant for which they were designed.

[Seroprevalence of anti-SARS-CoV-2 IgG antibodies in resident physicians]

Background

COVID-19 pandemic spread around the world swiftly; there are several diagnostic strategies available. Health workers, especially medical residents (MR), are a high-risk population for acquiring this infection.

Objective

To estimate the seroprevalence of antibodies against SARS-CoV-2 and the associated factors in MR of a third level hospital.

Material and methods

330 MR from different specialties were evaluated with a questionnaire and collection of blood samples for analysis by microparticle chemiluminescent immunoassay. The prevalence of previous infection was defined by seropositivity of these antibodies. Descriptive statistics and concordance between the RT-PCR tests and the presence of anti-SARS-CoV-2 IgG were used.

Results

Of 330 MR, 84.5% actively participated in COVID patient care. One out of 3 reported symptoms of COVID-19; in 67.6% the possible site of infection was a hospital setting not associated with the COVID area. Out of 71 symptomatic subjects, 61.9% underwent RT-PCR against SARS-CoV-2; 20 were positive. In 15.8% of the total, the presence of anti-SARS-CoV-2 IgG antibodies was determined. Only 1 out of 3 subjects with a positive PCR had antibodies, and 11.3% of the cases, even with a positive RT-PCR test, did not develop humoral immunity.

Conclusions

The seroprevalence was lower than that reported at the national level, potentially due to protection measures. The main risk factor was contact with the virus in areas of the hospital not related to COVID, making it imperative to reinforce security protocols in those spaces.

A pseudovirus-based platform to measure neutralizing antibodies in Mexico using SARS-CoV-2 as proof-of-concept

The gold-standard method to evaluate a functional antiviral immune response is to titer neutralizing antibodies (NAbs) against a viral pathogen. This is historically performed using an in vitro assay of virus-mediated infection, which requires BSL-3 facilities. As these are insufficient in Latin American countries, including Mexico, scant information is obtained locally about viral pathogens NAb, using a functional assay. An alternative solution to using a BSL-3 assay with live virus is to use a BSL-2-safe assay with a non-replicative pseudovirus. Pseudoviral particles can be engineered to display a selected pathogen's entry protein on their surface, and to deliver a reporter gene into target cells upon transduction. Here we comprehensively describe the first development of a BSL-2 safe NAbs-measuring functional assay in Mexico, based on the production of pseudotyped lentiviral particles. As proof-of-concept, the assay is based on Nanoluc luciferase-mediated luminescence measurements from target cells transduced with SARS-CoV-2 Spike-pseudotyped lentiviral particles. We applied the optimized assay in a BSL-2 facility to measure NAbs in 65 serum samples, which evidenced the assay with 100% sensitivity, 86.6% specificity and 96% accuracy. Overall, this is the first report of a BSL-2 safe pseudovirus-based functional assay developed in Mexico to measure NAbs, and a cornerstone methodology necessary to measure NAbs with a functional assay in limited resources settings.

Decrease in COVID-19 adverse outcomes in adults during the Delta and Omicron SARS-CoV-2 waves, after vaccination in Mexico

Mexico, one of the countries severely affected by COVID-19, accumulated more than 5. 1 all-cause excess deaths/1,000 inhabitants and 2.5 COVID-19 confirmed deaths/1,000 inhabitants, in 2 years. In this scenario of high SARS-CoV-2 circulation, we analyzed the effectiveness of the country's vaccination strategy that used 7 different vaccines from around the world, and focused on vaccinating the oldest population first. We analyzed the national dataset published by Mexican health authorities, as a retrospective cohort, separating cases, hospitalizations, deaths and excess deaths by wave and age group. We explored if the vaccination strategy was effective to limit severe COVID-19 during the active outbreaks caused by Delta and Omicron variants. Vaccination of the eldest third of the population reduced COVID-19 hospitalizations, deaths and excess deaths by 46-55% in the third wave driven by Delta SARS-CoV-2. These adverse outcomes dropped 74-85% by the fourth wave driven by Omicron, when all adults had access to vaccines. Vaccine access for the pregnant resulted in 85-90% decrease in COVID-19 fatalities in pregnant individuals and 80% decrease in infants 0 years old by the Omicron wave. In contrast, in the rest of the pediatric population that did not access vaccination before the period analyzed, COVID-19 hospitalizations increased >40% during the Delta and Omicron waves. Our analysis suggests that the vaccination strategy in Mexico has been successful to limit population mortality and decrease severe COVID-19, but children in Mexico still need access to SARS-CoV-2 vaccines to limit severe COVID-19, in particular those 1-4 years old.

COVID-19 in Latin America and the Caribbean: Two years of the pandemic

Worldwide, nations have struggled during the coronavirus disease 2019 (COVID-19) pandemic. However, Latin America and the Caribbean faced an unmatched catastrophic toll. As of March 2022, the region has reported approximately 15% of cases and 28% of deaths worldwide. Considering the relatively late arrival of SARS-CoV-2, several factors in the region were determinants of the humanitarian crisis that ensued. Pandemic unpreparedness, fragile healthcare systems, forthright inequalities, and poor governmental support facilitated the spread of the virus throughout the region. Moreover, reliance on repurposed and ineffective drugs such as hydroxychloroquine and ivermectin-to treat or prevent COVID-19-was publicised through misinformation and created a false sense of security and poor adherence to social distancing measures. While there were hopes that herd immunity could be achieved after the region's disastrous first peak, the emergence of the Gamma, Lambda, and Mu variants made this unattainable. This review explores how Latin America and the Caribbean fared during the first 2 years of the pandemic, and how, despite all the challenges, the region became a global leader in COVID-19 vaccination, with 63% of its population fully vaccinated.

Dominance of Three Sublineages of the SARS-CoV-2 Delta Variant in Mexico

In this study, we analyzed the sequences of SARS-CoV-2 isolates of the Delta variant in Mexico, which has completely replaced other previously circulating variants in the country due to its transmission advantage. Among all the Delta sublineages that were detected, 81.5 % were classified as AY.20, AY.26, and AY.100. According to publicly available data, these only reached a world prevalence of less than 1%, suggesting a possible Mexican origin. The signature mutations of these sublineages are described herein, and phylogenetic analyses and haplotype networks are used to track their spread across the country. Other frequently detected sublineages include AY.3, AY.62, AY.103, and AY.113. Over time, the main sublineages showed different geographical distributions, with AY.20 predominant in Central Mexico, AY.26 in the North, and AY.100 in the Northwest and South/Southeast. This work describes the circulation, from May to November 2021, of the primary sublineages of the Delta variant associated with the third wave of the COVID-19 pandemic in Mexico and highlights the importance of SARS-CoV-2 genomic surveillance for the timely identification of emerging variants that may impact public health.

Seroepidemiology of SARS-CoV-2 in healthcare personnel working at the largest tertiary COVID-19 referral hospitals in Mexico City

Introduction

We performed a longitudinal SARS-CoV-2 seroepidemiological study in healthcare personnel of the two largest tertiary COVID-19 referral hospitals in Mexico City.

Methods

All healthcare personnel, including staff physicians, physicians in training, nurses, laboratory technicians, researchers, students, housekeeping, maintenance, security, and administrative staff were invited to voluntarily participate, after written informed consent. Participants answered a computer-assisted self-administered interview and donated blood samples for antibody testing every three weeks from October 2020 to June 2021.

Results

A total of 883 participants (out of 3639 registered employees) contributed with at least one blood sample. The median age was 36 years (interquartile range: 28–46) and 70% were women. The most common occupations were nurse (28%), physician (24%), and administrative staff (22%). Two hundred and ninety participants (32.8%) had a positive-test result in any of the visits, yielding an overall adjusted prevalence of 33.5% for the whole study-period. Two hundred and thirty-five positive tests were identified at the baseline visit (prevalent cases), the remaining 55 positive tests were incident cases. Prevalent cases showed associations with both occupational (institution 2 vs. 1: adjusted odds ratio [aOR] = 2.24, 95% confidence interval [CI]: 1.54–3.25; laboratory technician vs. physician: aOR = 4.38, 95% CI: 1.75–10.93) and community (municipality of residence Xochimilco vs. Tlalpan: aOR = 2.03, 95% CI: 1.09–3.79) risk-factors. The incidence rate was 3.0 cases per 100 person-months. Incident cases were associated with community-acquired risk, due to contact with suspect/confirmed COVID-19 cases (HR = 2.45, 95% CI: 1.21–5.00).

Conclusions

We observed that between October 2020 and June 2021, healthcare workers of the two largest tertiary COVID-19 referral centers in Mexico City had similar level of exposure to SARS-CoV-2 than the general population. Most variables associated with exposure in this setting pointed toward community rather than occupational risk. Our observations are consistent with successful occupational medicine programs for SARS-CoV-2 infection control in the participating institutions but suggest the need to strengthen mitigation strategies in the community.

Seroconversion dynamic and SARS-CoV-2 seropositivity in unvaccinated population during the first and second outbreaks in Mexico

Serosurveillance helps establish reopening guidelines and determine the immunity levels in different populations to reach herd immunity. Then, there is an urgent need to estimate seroprevalence population wide. In Mexico, information about COVID-19 cases and related deaths is scarce. Also, there is no official serosurveillance, limiting our knowledge of the impact of the SARS-CoV-2 pandemic. Here, we report the prevalence of anti-SARS-CoV-2 antibodies in 522,690 unvaccinated people from July 5th to December 31st, 2020. The overall seroprevalence was 32.8% and highest in adults aged 30–39 years (38.5%) than people under 20 years (33.0%) or older (28.9%). Moreover, in a cohort of 1655 individuals confirmed COVID-19 by PCR, we found that symptomatic people (HR = 2.56) increased seroconversion than presymptomatic. Also, we identified that the most discriminative symptoms for COVID-19 that could predict seroconversion were anosmia and ageusia (HR = 1.70), fever, myalgia/arthralgia, and cough (HR = 1.75). Finally, we found that obese people had lower seroconversion (HR = 0.53) than healthy people, but the opposite happens in diabetic people (HR = 1.39). These findings reveal that around one-third of Mexican outpatients had anti-SARS-CoV-2 antibodies before vaccination. Also, some symptoms improve empirically COVID-19 diagnosis and seroconversion. This information could help fine-tune vaccination schemes and the reopening and back-to-work algorithms.

The impact of the COVID-19 pandemic among migrants in shelters in Tijuana, Baja California, Mexico

INTRODUCTION

Migrants, especially those in temporary accommodations like camps and shelters, might be a vulnerable population during the COVID-19 pandemic, but little is known about the impact of the pandemic in these settings in low-income and middle-income countries. We assessed SARS-CoV-2 seropositivity and RNA prevalence, the correlates of seropositivity (emphasising socially determined conditions), and the socioeconomic impacts of the pandemic among migrants living in shelters in Tijuana, a city on the Mexico-US border.

METHODS

We conducted a cross-sectional, non-probability survey of migrants living in shelters in Tijuana in November-December 2020 and February-April 2021. Participants completed a questionnaire and provided anterior nasal swab and blood samples for detection of SARS-CoV-2 RNA and antibodies (IgG and IgM), respectively. We explored whether SARS-CoV-2 infection was associated with sociodemographic and migration-related variables, access to sanitation, protective behaviours and health-related factors.

RESULTS

Overall, 481 participants were enrolled, 67.7% from Northern Central America, 55.3% women, mean age 33.2 years. Seven (1.5%) participants had nasal swabs positive for SARS-CoV-2 RNA and 53.0% were SARS-CoV-2 seropositive. Avoiding public transportation (OR 0.59, 95% CI 0.39 to 0.90) and months living in Tijuana (OR 1.06, 95% CI 1.02 to 1.10) were associated with seropositivity. Sleeping on the streets or other risky places and having diabetes were marginally associated with seropositivity. Most participants (90.2%) had experienced some socioeconomic impact of the pandemic (eg, diminished income, job loss).

CONCLUSION

Compared with results from other studies conducted in the general population in Mexico at a similar time, migrants living in shelters were at increased risk of acquiring SARS-CoV-2, and they suffered considerable adverse socioeconomic impacts as a consequence of the pandemic. Expanded public health and other social support systems are needed to protect migrants from COVID-19 and reduce health inequities.

Prevalence of SARS-CoV-2 infection in Baja California, Mexico: Findings from a community-based survey in February 2021 in the Mexico-United States border

Between March 2020 and February 2021, the state of Baja California, Mexico, which borders the United States, registered 46,118 confirmed cases of COVID-19 with a mortality rate of 238.2 deaths per 100,000 residents. Given limited access to testing, the population prevalence of SARS-CoV-2 infection is unknown. The objective of this study is to estimate the seroprevalence and real time polymerase chain reaction (RT-PCR) prevalence of SARS-CoV-2 infection in the three most populous cities of Baja California prior to scale-up of a national COVID-19 vaccination campaign. Probabilistic three-stage clustered sampling was used to conduct a population-based household survey of residents five years and older in the three cities. RT-PCR testing was performed on nasopharyngeal swabs and SARS-CoV-2 seropositivity was determined by IgG antibody testing using fingerstick blood samples. An interviewer-administered questionnaire assessed participants' knowledge, attitudes, and preventive practices regarding COVID-19. In total, 1,126 individuals (unweighted sample) were surveyed across the three cities. Overall prevalence of SARS-CoV-2 infection by RT-PCR was 7.8% (95% CI 5.5-11.0) and IgG seroprevalence was 21.1% (95% CI 17.4-25.2). There was no association between border crossing in the past 6 months and SARS-CoV-2 prevalence (unadjusted OR 0.40, 95%CI 0.12-1.30). While face mask use and frequent hand washing were common among participants, quarantine or social isolation at home to prevent infection was not. Regarding vaccination willingness, 30.4% (95% CI 24.4-3 7.1) of participants said they were very unlikely to get vaccinated. Given the high prevalence of active SARS-CoV-2 infection in Baja California at the end of the first year of the pandemic, combined with its low seroprevalence and the considerable proportion of vaccine hesitancy, this important area along the Mexico-United States border faces major challenges in terms of health literacy and vaccine uptake, which need to be further explored, along with its implications for border restrictions in future epidemics.

COVID-19 underreporting and its impact on vaccination strategies

BACKGROUND

Underreporting cases of infectious diseases poses a major challenge in the analysis of their epidemiological characteristics and dynamical aspects. Without accurate numerical estimates it is difficult to precisely quantify the proportions of severe and critical cases, as well as the mortality rate. Such estimates can be provided for instance by testing the presence of the virus. However, during an ongoing epidemic, such tests' implementation is a daunting task. This work addresses this issue by presenting a methodology to estimate underreported infections based on approximations of the stable rates of hospitalization and death.

METHODS

We present a novel methodology for the stable rate estimation of hospitalization and death related to the Corona Virus Disease 2019 (COVID-19) using publicly available reports from various distinct communities. These rates are then used to estimate underreported infections on the corresponding areas by making use of reported daily hospitalizations and deaths. The impact of underreporting infections on vaccination strategies is estimated under different disease-transmission scenarios using a Susceptible-Exposed-Infective-Removed-like (SEIR) epidemiological model.

RESULTS

For the considered locations, during the period of study, the estimations suggest that the number of infected individuals could reach 30% of the population of these places, representing, in some cases, more than six times the observed numbers. These results are in close agreement with estimates from independent seroprevalence studies, thus providing a strong validation of the proposed methodology. Moreover, the presence of large numbers of underreported infections can reduce the perceived impact of vaccination strategies in reducing rates of mortality and hospitalization.

CONCLUSIONS

pBy using the proposed methodology and employing a judiciously chosen data analysis implementation, we estimate COVID-19 underreporting from publicly available data. This leads to a powerful way of quantifying underreporting impact on the efficacy of vaccination strategies. As a byproduct, we evaluate the impact of underreporting in the designing of vaccination strategies.

Prevalence of IgG antibodies induced by the SARS-COV-2 virus in asymptomatic adults in Nuevo Leon, Mexico

Population‐based immunoglobulin G (IgG) seroprevalence studies in asymptomatic individuals in Latin America are scarce. The objective of the study was to estimate the prevalence and geographic distribution of IgG antibodies induced by natural SARS‐CoV‐2 infection in asymptomatic adults, 5–8 months after the first case was reported in a northeastern state of Mexico. This was a population‐based cross‐sectional study carried out in Nuevo Leon during August–November 2020. Individuals ≥18 years with no previous diagnosis or symptoms suggestive of COVID‐19 were consecutively screened in one of the busiest subway stations. Also, a search for eligible individuals was done from house‐to‐house, after selecting densely populated geographic sectors of each of the municipalities of the metropolitan area (n = 4495). The IgG antibodies to SARS‐CoV‐2 nucleocapsid protein were analyzed. The IgG antibody positivity rate was 27.1% (95% confidence interval [CI]: 25.8, 28.4); there were no differences by sex or age (p > 0.05). Analysis by month showed a gradual increase from 11.9% (August) to 31.9% (November); Week 39 had the highest positivity rate (42.2%, 95% CI: 34.2, 50.7). Most people did not have evidence of previous SARS‐CoV‐2 infection. Preventive measures and promotion of the COVID‐19 vaccine should be strengthened.