SARS-CoV-2 Infections and Hospitalizations Among Persons Aged ≥16 Years, by Vaccination Status - Los Angeles County, California, May 1-July 25, 2021

Abstract

COVID-19 vaccines fully approved or currently authorized for use through Emergency Use Authorization from the Food and Drug Administration are critical tools for controlling the COVID-19 pandemic; however, even with highly effective vaccines, a proportion of fully vaccinated persons will become infected with SARS-CoV-2, the virus that causes COVID-19 (1). To characterize postvaccination infections, the Los Angeles County Department of Public Health (LACDPH) used COVID-19 surveillance and California Immunization Registry 2 (CAIR2) data to describe age-adjusted infection and hospitalization rates during May 1-July 25, 2021, by vaccination status. Whole genome sequencing (WGS)-based SARS-CoV-2 lineages and cycle threshold (Ct) values from qualitative reverse transcription-polymerase chain reaction (RT-PCR) for two SARS-CoV-2 gene targets, including the nucleocapsid (N) protein gene region and the open reading frame 1 ab (ORF1ab) polyprotein gene region,* were reported for a convenience sample of specimens. Among 43,127 reported SARS-CoV-2 infections in Los Angeles County residents aged ≥16 years, 10,895 (25.3%) were in fully vaccinated persons, 1,431 (3.3%) were in partially vaccinated persons, and 30,801 (71.4%) were in unvaccinated persons. Much lower percentages of fully vaccinated persons infected with SARS-CoV-2 were hospitalized (3.2%), were admitted to an intensive care unit (0.5%), and required mechanical ventilation (0.2%) compared with partially vaccinated persons (6.2%, 1.0%, and 0.3%, respectively) and unvaccinated persons (7.6%, 1.5%, and 0.5%, respectively) (p<0.001 for all comparisons). On July 25, the SARS-CoV-2 infection rate among unvaccinated persons was 4.9 times and the hospitalization rate was 29.2 times the rates among fully vaccinated persons. During May 1-July 25, the percentages of B.1.617.2 (Delta) variant infections estimated from 6,752 samples with lineage data increased among fully vaccinated persons (from 8.6% to 91.2%), partially vaccinated persons (from 0% to 88.1%), and unvaccinated persons (from 8.2% to 87.1%). In May, there were differences in median Ct values by vaccination status; however, by July, no differences were detected among specimens from fully vaccinated, partially vaccinated, and unvaccinated persons by gene targets. These infection and hospitalization rate data indicate that authorized vaccines were protective against SARS-CoV-2 infection and severe COVID-19 during a period when transmission of the Delta variant was increasing. Efforts to increase COVID-19 vaccination, in coordination with other prevention strategies, are critical to preventing COVID-19-related hospitalizations and deaths.

FIGURE 1

Age-adjusted rolling 7-day SARS-CoV-2 infection and hospitalization rates, by vaccination status — Los Angeles County, California, May 1–July 25, 2021 * Rolling 7-day incidence was calculated by summing the total number of persons or hospitalizations during a 7-day period and dividing by the total population at the end of the 7-day period. ^† Persons were considered fully vaccinated ≥14 days after receipt of the second dose in a 2-dose series (Pfizer-BioNTech or Moderna COVID-19 vaccines) or after 1 dose of the single-dose Janssen (Johnson & Johnson) COVID-19 vaccine; partially vaccinated ≥14 days after receipt of the first dose and <14 days after the second dose in a 2-dose series; and unvaccinated <14 days receipt of the first dose of a 2-dose series or 1 dose of the single-dose vaccine or if no vaccination registry data were available.

FIGURE 2

SARS-CoV-2 whole genome sequencing lineage results and reverse transcription–polymerase chain reaction cycle threshold values for two gene targets, by vaccination status and month — Los Angeles County, California, May 1–July 25, 2021 Abbreviations: Ct = cycle threshold; N = nucleocapsid; ORF1ab = open reading frame 1 ab; RT-PCR = reverse transcription–polymerase chain reaction; SC2N = SARS-CoV-2 nucleocapsid. * SARS-CoV-2 infections among Los Angeles County residents aged ≥16 years with whole genome sequencing lineage results (n = 6,752) for fully vaccinated (n = 1,667), partially vaccinated (n = 198), and unvaccinated (n = 4,887) persons. ^† Whiskers represent minimum and maximum observations; top of box represents the third quartile, bottom represents the first quartile, and box height represents the interquartile range. The midline is the median. ^§ Ct values are correlated with the amount of viral nucleic acid present. Gene targets for RT-PCR testing included the N protein gene region and the ORF1ab polyprotein gene region. The N gene targets were analyzed separately for two laboratories because Ct values are not directly comparable across different testing laboratories; these N gene targets were designated SC2N and N to differentiate between the two participating laboratory partners. Gene targets were selected based on testing platforms used by Los Angeles County Department of Public Health laboratory partners. Analysis of SC2N Ct values is restricted to a Fulgent test result with a Ct value on the same day as person’s first positive RT-PCR test result; SC2N gene target values (n = 5,179) are stratified for fully vaccinated (n = 1,248), partially vaccinated (n = 151), and unvaccinated (n = 3,780) persons. Analysis of ORF1ab and N Ct values is restricted to a Valencia Branch Laboratory test result with a Ct value on the same day as person’s first positive RT-PCR test result. ORF1ab (n = 1,041) and N (n = 1,062) gene target values are stratified for fully vaccinated (n = 289 and n = 297, respectively), partially vaccinated (n = 36 and n = 41, respectively), and unvaccinated (n = 716 and n = 724, respectively) persons. ^¶ Persons were considered fully vaccinated ≥14 days after receipt of the second dose in a 2-dose series (Pfizer-BioNTech or Moderna COVID-19 vaccines) or after 1 dose of the single-dose Janssen (Johnson & Johnson) COVID-19 vaccine; partially vaccinated ≥14 days after receipt of the first dose and <14 days after the second dose in a 2-dose series; and unvaccinated <14 days receipt of the first dose of a 2-dose series or 1 dose of the single-dose vaccine or if no vaccination registry data were available.