Prevalence and Clinical Profile of Severe Acute Respiratory Syndrome Coronavirus 2 Infection among Farmworkers,California, USA,June–November 2020

During the ongoing coronavirus disease (COVID-19) pandemic, farmworkers in the United States are considered essential personnel and continue in-person work. We conducted prospective surveillance for severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and antibody prevalence among farmworkers in Salinas Valley, California, during June 15–November 30, 2020. We observed 22.1% (1,514/6,864) positivity for SARS-CoV-2 infection among farmworkers compared with 17.2% (1,255/7,305) among other adults from the same communities (risk ratio 1.29, 95% CI 1.20–1.37). In a nested study enrolling 1,115 farmworkers, prevalence of current infection was 27.7% among farmworkers reporting >1 COVID-19 symptom and 7.2% among farmworkers without symptoms (adjusted odds ratio 4.16, 95% CI 2.85–6.06). Prevalence of SARS-CoV-2 antibodies increased from 10.5% (95% CI 6.0%–18.4%) during July 16–August 31 to 21.2% (95% CI 16.6%–27.4%) during November 1–30. High SARS-CoV-2 infection prevalence among farmworkers underscores the need for vaccination and other preventive interventions.     

Effectiveness of 3 COVID-19 Vaccines in Preventing SARS-CoV-2 Infections,January–May 2021, Aragon,Spain

Reducing severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) transmission is a worldwide challenge; widespread vaccination could be one strategy for control. We conducted a prospective, population-based cohort study of 964,258 residents of Aragon, Spain, during December 2020–May 2021. We used the Cox proportional-hazards model with vaccination status as the exposure condition to estimate the effectiveness of 3 coronavirus disease vaccines in preventing SARS-CoV-2 infection. Pfizer-BioNTech had 20.8% (95% CI 11.6%–29.0%) vaccine effectiveness (VE) against infection after 1 dose and 70.0% (95% CI 65.3%–74.1%) after 2 doses, Moderna had 52.8% (95% CI 30.7%–67.8%) VE after 1 dose and 70.3% (95% CI 52.2%–81.5%) after 2 doses, and Oxford-AstraZeneca had 40.3% (95% CI 31.8%–47.7%) VE after 1 dose. All estimates were lower than those from previous studies. Results imply that, although high vaccination coverage remains critical to protect people from disease, it will be difficult to effectively minimize transmission opportunities.     

Seroprevalence of SARS-CoV-2 in Guilan Province,Iran, April 2020

We determined the seroprevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in an affected area in northern Iran in April 2020. Antibodies to SARS-CoV-2 were detected in 528 persons by using rapid tests. Adjusted prevalence of SARS-CoV-2 seropositivity was 22.2% (95% CI 16.4%–28.5%).     

Seroprevalence of Severe Acute Respiratory Syndrome Coronavirus 2 IgG in Juba,South Sudan, 2020

Relatively few coronavirus disease cases and deaths have been reported from sub-Saharan Africa, although the extent of its spread remains unclear. During August 10–September 11, 2020, we recruited 2,214 participants for a representative household-based cross-sectional serosurvey in Juba, South Sudan. We found 22.3% of participants had severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) receptor binding domain IgG titers above prepandemic levels. After accounting for waning antibody levels, age, and sex, we estimated that 38.3% (95% credible interval 31.8%–46.5%) of the population had been infected with SARS-CoV-2. At this rate, for each PCR–confirmed SARS-CoV-2 infection reported by the Ministry of Health, 103 (95% credible interval 86–126) infections would have been unreported, meaning SARS-CoV-2 has likely spread extensively within Juba. We also found differences in background reactivity in Juba compared with Boston, Massachusetts, USA, where the immunoassay was validated. Our findings underscore the need to validate serologic tests in sub-Saharan Africa populations.     

Estimating COVID-19 Vaccine Effectiveness for Skilled Nursing Facility Healthcare Personnel,California, USA

We estimated real-world vaccine effectiveness among skilled nursing facility healthcare personnel who were regularly tested for SARS-CoV-2 infection in California, USA, during January‒March 2021. Vaccine effectiveness for fully vaccinated healthcare personnel was 73.3% (95% CI 57.5%–83.3%). We observed high real-world vaccine effectiveness in this population.     

Direct and Indirect Effectiveness of mRNA Vaccination against Severe Acute Respiratory Syndrome Coronavirus 2 in Long-Term Care Facilities,Spain

We conducted a registries-based cohort study of long-term care facility residents >65 years of age offered vaccination against severe acute respiratory syndrome coronavirus 2 before March 10, 2021, in Spain. Risk for infection in vaccinated and nonvaccinated persons was compared with risk in the same persons in a period before the vaccination campaign, adjusted by daily-varying incidence and reproduction number. We selected 299,209 persons; 99.0% had >1 dose, 92.6% had 2 doses, and 99.8% of vaccines were Pfizer/BioNTech (BNT162b2). For vaccinated persons with no previous infection, vaccine effectiveness was 81.8% (95% CI 81.0%–82.7%), and 11.6 (95% CI 11.3–11.9) cases were prevented per 10,000 vaccinated/day. In those with previous infection, effectiveness was 56.8% (95% CI 47.1%–67.7%). In nonvaccinated residents with no previous infection, risk decreased by up to 81.4% (95% CI 73.3%–90.3%). Our results confirm vaccine effectiveness in this population and suggest indirect protection in nonvaccinated persons.     

Effectiveness of BNT162b2 Vaccine Booster against SARS-CoV-2 Infection and Breakthrough Complications,Israel

We estimated vaccine effectiveness (VE) of the BNT162b2 (Pfizer-BioNTech, https://www.pfizer.com) booster dose against SARS-CoV-2 infection and reduction of complications (hospitalization, severe disease, and death) among breakthrough cases in persons in Israel >16 years of age for <20 weeks. VE estimates reached 96.8% (95% CI 96.0%–97.5%) for persons 16–59 years of age and 93.1% (95% CI 91.8%–94.2%) for persons >60 years of age on week 3. VE estimates remained at these levels for 8 weeks in the 16–59 age group and 11 weeks in those >60. A slow decline followed, becoming more pronounced in the last 2–3 weeks of evaluation. Estimates in the last week of evaluation were 77.6% (95% CI 68.4%–84.2%) and 61.3% (52.5%–68.4%) for persons 16–59 years and >60 years, respectively. The more pronounced VE decline coincided with rapid increase in Omicron variant activity. Rate reduction of breakthrough complications remained moderate to high throughout the evaluation.     

Severe Acute Respiratory Syndrome Coronavirus 2 Seropositivity among Healthcare Personnel in Hospitals and Nursing Homes,Rhode Island,USA, July–August 2020

Healthcare personnel are recognized to be at higher risk for infection with severe acute respiratory syndrome coronavirus 2. We conducted a serologic survey in 15 hospitals and 56 nursing homes across Rhode Island, USA, during July 17–August 28, 2020. Overall seropositivity among 9,863 healthcare personnel was 4.6% (95% CI 4.2%–5.0%) but varied 4-fold between hospital personnel (3.1%, 95% CI 2.7%–3.5%) and nursing home personnel (13.1%, 95% CI 11.5%–14.9%). Within nursing homes, prevalence was highest among personnel working in coronavirus disease units (24.1%; 95% CI 20.6%–27.8%). Adjusted analysis showed that in hospitals, nurses and receptionists/medical assistants had a higher likelihood of seropositivity than physicians. In nursing homes, nursing assistants and social workers/case managers had higher likelihoods of seropositivity than occupational/physical/speech therapists. Nursing home personnel in all occupations had elevated seropositivity compared with hospital counterparts. Additional mitigation strategies are needed to protect nursing home personnel from infection, regardless of occupation.     

Prevalence of SARS-CoV-2 Antibodies in First Responders and Public Safety Personnel,New York City,New York,USA, May–July 2020

We conducted a serologic survey in public service agencies in New York City, New York, USA, during May–July 2020 to determine prevalence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection among first responders. Of 22,647 participants, 22.5% tested positive for SARS-CoV-2–specific antibodies. Seroprevalence for police and firefighters was similar to overall seroprevalence; seroprevalence was highest in correctional staff (39.2%) and emergency medical technicians (38.3%) and lowest in laboratory technicians (10.1%) and medicolegal death investigators (10.8%). Adjusted analyses demonstrated association between seropositivity and exposure to SARS-CoV-2–positive household members (adjusted odds ratio [aOR] 3.52 [95% CI 3.19–3.87]), non-Hispanic Black race or ethnicity (aOR 1.50 [95% CI 1.33–1.68]), and severe obesity (aOR 1.31 [95% CI 1.05–1.65]). Consistent glove use (aOR 1.19 [95% CI 1.06–1.33]) increased likelihood of seropositivity; use of other personal protective equipment had no association. Infection control measures, including vaccination, should be prioritized for frontline workers.     

SARS-CoV-2 Seroprevalence among Healthcare,First Response,and Public Safety Personnel,Detroit Metropolitan Area,Michigan, USA,May–June 2020

To estimate seroprevalence of severe acute respiratory syndrome 2 (SARS-CoV-2) among healthcare, first response, and public safety personnel, antibody testing was conducted in emergency medical service agencies and 27 hospitals in the Detroit, Michigan, USA, metropolitan area during May–June 2020. Of 16,403 participants, 6.9% had SARS-CoV-2 antibodies. In adjusted analyses, seropositivity was associated with exposure to SARS-CoV-2–positive household members (adjusted odds ratio [aOR] 6.18, 95% CI 4.81–7.93) and working within 15 km of Detroit (aOR 5.60, 95% CI 3.98–7.89). Nurse assistants (aOR 1.88, 95% CI 1.24–2.83) and nurses (aOR 1.52, 95% CI 1.18–1.95) had higher likelihood of seropositivity than physicians. Working in a hospital emergency department increased the likelihood of seropositivity (aOR 1.16, 95% CI 1.002–1.35). Consistently using N95 respirators (aOR 0.83, 95% CI 0.72–0.95) and surgical facemasks (aOR 0.86, 95% CI 0.75–0.98) decreased the likelihood of seropositivity.     

Reassessing Reported Deaths and Estimated Infection Attack Rate during the First 6 Months of the COVID-19 Epidemic,Delhi, India

India reported >10 million coronavirus disease (COVID-19) cases and 149,000 deaths in 2020. To reassess reported deaths and estimate incidence rates during the first 6 months of the epidemic, we used a severe acute respiratory syndrome coronavirus 2 transmission model fit to data from 3 serosurveys in Delhi and time-series documentation of reported deaths. We estimated 48.7% (95% credible interval 22.1%–76.8%) cumulative infection in the population through the end of September 2020. Using an age-adjusted overall infection fatality ratio based on age-specific estimates from mostly high-income countries, we estimated that just 15.0% (95% credible interval 9.3%–34.0%) of COVID-19 deaths had been reported, indicating either substantial underreporting or lower age-specific infection-fatality ratios in India than in high-income countries. Despite the estimated high attack rate, additional epidemic waves occurred in late 2020 and April–May 2021. Future dynamics will depend on the duration of natural and vaccine-induced immunity and their effectiveness against new variants.     

IgG Seroconversion and Pathophysiology in Severe Acute Respiratory Syndrome Coronavirus 2 Infection

We investigated the dynamics of seroconversion in severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. During March 29–May 22, 2020, we collected serum samples and associated clinical data from 177 persons in London, UK, who had SARS-CoV-2 infection. We measured IgG against SARS-CoV-2 and compared antibody levels with patient outcomes, demographic information, and laboratory characteristics. We found that 2.0%–8.5% of persons did not seroconvert 3–6 weeks after infection. Persons who seroconverted were older, were more likely to have concurrent conditions, and had higher levels of inflammatory markers. Non-White persons had higher antibody concentrations than those who identified as White; these concentrations did not decline during follow-up. Serologic assay results correlated with disease outcome, race, and other risk factors for severe SARS-CoV-2 infection. Serologic assays can be used in surveillance to clarify the duration and protective nature of humoral responses to SARS-CoV-2 infection.     

Population-Based Serosurvey for Severe Acute Respiratory Syndrome Coronavirus 2 Transmission,Chennai, India

We conducted a cross-sectional survey to estimate the seroprevalence of IgG against severe acute respiratory syndrome coronavirus 2 in Chennai, India. Among 12,405 serum samples tested, weighted seroprevalence was 18.4% (95% CI 14.8%–22.6%). These findings indicate most of the population of Chennai is still susceptible to this virus.     

Effects of Social Distancing Measures during the First Epidemic Wave of Severe Acute Respiratory Syndrome Infection,Greece

Greece imposed a nationwide lockdown in March 2020 to mitigate transmission of severe acute respiratory syndrome coronavirus 2 during the first epidemic wave. We conducted a survey on age-specific social contact patterns to assess effects of physical distancing measures and used a susceptible-exposed-infectious-recovered model to simulate the epidemic. Because multiple distancing measures were implemented simultaneously, we assessed their overall effects and the contribution of each measure. Before measures were implemented, the estimated basic reproduction number (R₀) was 2.38 (95% CI 2.01–2.80). During lockdown, daily contacts decreased by 86.9% and R₀ decreased by 81.0% (95% credible interval [CrI] 71.8%–86.0%); each distancing measure decreased R₀ by 10%–24%. By April 26, the attack rate in Greece was 0.12% (95% CrI 0.06%–0.26%), one of the lowest in Europe, and the infection fatality ratio was 1.12% (95% CrI 0.55%–2.31%). Multiple social distancing measures contained the first epidemic wave in Greece.     

Antibody Responses 8 Months after Asymptomatic or Mild SARS-CoV-2 Infection

Waning humoral immunity in coronavirus disease patients has raised concern over usefulness of serologic testing. We investigated antibody responses of 58 persons 8 months after asymptomatic or mildly symptomatic infection with severe acute respiratory syndrome coronavirus 2. For 3 of 4 immunoassays used, seropositivity rates were high (69.0%–91.4%).     

Analyzing and Modeling the Spread of SARS-CoV-2 Omicron Lineages BA.1 and BA.2, France,September 2021–February 2022

We analyzed 324,734 SARS-CoV-2 variant screening tests from France enriched with 16,973 whole-genome sequences sampled during September 1, 2021–February 28, 2022. Results showed the estimated growth advantage of the Omicron variant over the Delta variant to be 105% (95% CI 96%–114%) and that of the BA.2 lineage over the BA.1 lineage to be 49% (95% CI 44%–52%). Quantitative PCR cycle threshold values were consistent with an increased ability of Omicron to generate breakthrough infections. Epidemiologic modeling shows that, in spite of its decreased virulence, the Omicron variant can generate important critical COVID-19 activity in hospitals in France. The magnitude of the BA.2 wave in hospitals depends on the level of relaxing of control measures but remains lower than that of BA.1 in median scenarios.     

Social Distancing,Mask Use,and Transmission of Severe Acute Respiratory Syndrome Coronavirus 2, Brazil,April–June 2020

We assessed the associations of social distancing and mask use with symptomatic, laboratory-confirmed severe acute respiratory syndrome coronavirus 2 infection in Porto Alegre, Brazil. We conducted a population-based case-control study during April–June 2020. Municipal authorities furnished case-patients, and controls were taken from representative household surveys. In adjusted logistic regression analyses of 271 case-patients and 1,396 controls, those reporting moderate to greatest adherence to social distancing had 59% (odds ratio [OR] 0.41, 95% CI 0.24–0.70) to 75% (OR 0.25, 95% CI 0.15–0.42) lower odds of infection. Lesser out-of-household exposure (vs. going out every day all day) reduced odds from 52% (OR 0.48, 95% CI 0.29–0.77) to 75% (OR 0.25, 95% CI 0.18–0.36). Mask use reduced odds of infection by 87% (OR 0.13, 95% CI 0.04–0.36). In conclusion, social distancing and mask use while outside the house provided major protection against symptomatic infection.     

Cellular Immunity in COVID-19 Convalescents with PCR-Confirmed Infection but with Undetectable SARS-CoV-2–Specific IgG

We investigated immune responses against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) among a group of convalescent, potential blood donors in Germany who had PCR-confirmed SARS-CoV-2 infection. Sixty days after onset of symptoms, 13/78 (17%) study participants had borderline or negative results to an ELISA detecting IgG against the S1 protein of SARS-CoV-2. We analyzed participants with PCR-confirmed infection who had strong antibody responses (ratio >3) as positive controls and participants without symptoms of SARS-CoV-2 infection and without household contact with infected patients as negative controls. Using interferon-γ ELISpot, we observed that 78% of PCR-positive volunteers with undetectable antibodies showed T cell immunity against SARS-CoV-2. We observed a similar frequency (80%) of T-cell immunity in convalescent donors with strong antibody responses but did not detect immunity in negative controls. We concluded that, in convalescent patients with undetectable SARS-CoV-2 IgG, immunity may be mediated through T cells.