COVID-19 vaccines are effective at preventing infection with SARS-CoV-2, the virus
that causes COVID-19, as well as severe COVID-19–associated outcomes in real-world
conditions (
1
,
2
). The risks for SARS-CoV-2 infection and COVID-19–associated hospitalization are
lower among fully vaccinated than among unvaccinated persons; this reduction is even
more pronounced among those who have received additional or booster doses (boosters)
(
3
,
4
). Although the B.1.1.529 (Omicron) variant spreads more rapidly than did earlier
SARS-CoV-2 variants, recent studies suggest that disease severity is lower for Omicron
compared with that associated with the B.1.617.2 (Delta) variant; but the high volume
of infections is straining the health care system more than did previous waves (
5
).*
,
†
The Los Angeles County (LAC) Department of Public Health (LACDPH) used COVID-19 surveillance
and California Immunization Registry 2 (CAIR2) data to describe age-adjusted 14-day
cumulative incidence and hospitalization rates during November 7, 2021–January 8,
2022, by COVID-19 vaccination status and variant predominance. For the 14-day period
ending December 11, 2021, the last week of Delta predominance, the incidence and hospitalization
rates among unvaccinated persons were 12.3 and 83.0 times, respectively, those of
fully vaccinated persons with a booster and 3.8 and 12.9 times, respectively, those
of fully vaccinated persons without a booster. These rate ratios were lower during
Omicron predominance (week ending January 8, 2022), with unvaccinated persons having
infection and hospitalization rates 3.6 and 23.0 times, respectively, those of fully
vaccinated persons with a booster and 2.0 and 5.3 times, respectively, those of fully
vaccinated persons without a booster. In addition, during the entire analytic period,
admission to intensive care units (ICUs), intubation for mechanical ventilation, and
death were more likely to occur among unvaccinated persons than among fully vaccinated
persons without or with a booster (p<0.001). Incidence and hospitalization rates were
consistently highest for unvaccinated persons and lowest for fully vaccinated persons
with a booster. Being up to date with COVID-19 vaccination is critical to protecting
against SARS-CoV-2 infection and associated hospitalization.
LACDPH conducted a cross-sectional analysis of LAC residents aged ≥18 years with laboratory-confirmed
SARS-CoV-2 infection (a positive SARS-CoV-2 result from a nucleic acid amplification
or antigen test) during November 7, 2021–January 8, 2022.
§
Persons were considered fully vaccinated ≥14 days after receipt of the final dose
in the primary series of a BNT162b2 (Pfizer-BioNTech), mRNA-1273 (Moderna), or Ad.26.COV2.S
(Janssen [Johnson & Johnson]) vaccine and considered unvaccinated if <14 days had
elapsed since receipt of the first dose in the primary series of an mRNA or Janssen
vaccine or if no matching immunization record was found in CAIR2.
¶
Fully vaccinated persons who received a booster were considered fully vaccinated with
a booster ≥14 days after the date of the booster.** Infections occurring in partially
vaccinated persons (persons who had received the first dose in a 2-dose series >14
days earlier, but who were either missing a second dose or <14 days had elapsed since
receipt of the second dose) were excluded because of small sample size.
††
COVID-19–associated hospitalizations were defined as hospital admissions occurring
≤14 days after the first laboratory-confirmed positive SARS-CoV-2 test result (
6
). Whole genome sequencing data from laboratories conducting routine genomic surveillance
for LAC were used to calculate weekly variant proportions.
§§
All available variant data were reported by date of specimen collection and used to
assess periods of predominance (>50% of sequenced specimens) for the Delta and Omicron
variants.
Demographic and clinical characteristics of SARS-CoV-2 infections were compared by
vaccination status using Pearson’s chi-square tests for categorical variables and
Kruskal-Wallis tests for medians. P-values <0.05 were considered statistically significant.
Age-adjusted rolling 14-day SARS-CoV-2 infection and hospitalization rates and rate
ratios among LAC residents aged ≥18 years were estimated by vaccination status using
2019 population estimates and standardized using the year 2000 U.S. standard population.
¶¶
Analyses were conducted using SAS (version 9.4; SAS Institute) and R (version 3.6.2;
R Foundation). This activity was determined by LACDPH’s Institutional Review Board
to be a surveillance activity necessary for public health work and therefore did not
require Institutional Review Board review.
From mid-August 2021 until the emergence of Omicron in November 2021, nearly 100%
of SARS-CoV-2 infections among LAC residents with sequenced specimens were caused
by the Delta variant. The earliest known Omicron variant infection in LAC was identified
in a specimen collected during the final week of November 2021. As Omicron emerged
in LAC, Delta prevalence decreased 95% during the week ending December 11. Omicron
became the predominant SARS-CoV-2 variant in LAC during the week ending December 18,
accounting for 57% of all sequenced specimens; Omicron prevalence continued to increase,
accounting for 99% of all sequenced specimens for the week ending January 8, 2022
(Supplementary Figure, https://stacks.cdc.gov/view/cdc/113859).
Among 422,966 reported SARS-CoV-2 infections in LAC residents aged ≥18 years during
November 7, 2021–January 8, 2022, a total of 141,928 (33.6%) were in unvaccinated
persons, 56,185 (13.3%) were in fully vaccinated persons with a booster, and 224,853
(53.2%) were in fully vaccinated persons without a booster (Table). Unvaccinated persons
were most likely to be hospitalized (2.8%), admitted to an ICU (0.5%), and require
intubation for mechanical ventilation (0.2%); these outcomes were less common in fully
vaccinated persons with a booster (0.7%, 0.08%, and 0.03%, respectively) and fully
vaccinated persons without a booster (1.0%, 0.12%, and 0.05%, respectively) (p<0.001).
Deaths were also more likely to occur among unvaccinated persons (0.3%) than among
fully vaccinated persons with a booster (0.07%) or without (0.08%) (p<0.001).
TABLE
Selected characteristics of cases of SARS-CoV-2 infection in residents aged ≥18 years
(N = 422,966), by vaccination status — Los Angeles County, California, November 7,
2021–January 8, 2022*
,
†
Characteristic
Vaccination status, no. (column %)
Unvaccinated
Fully vaccinated without booster
Fully vaccinated with booster
Total no. of cases (row %)
141,928 (33.6)
224,853 (53.2)
56,185 (13.3)
Median age, yrs (IQR)
35 (27–48)
36 (27–49)
46 (33–59)
18–29
48,940 (34.5)
74,352 (33.1)
9,523 (16.9)
30–49
61,380 (43.2)
97,771 (43.5)
22,649 (40.3)
50–64
22,338 (15.7)
40,680 (18.1)
14,580 (25.9)
65–79
7,253 (5.1)
9,796 (4.4)
7,960 (14.2)
≥80
2,017 (1.4)
2,254 (1.0)
1,473 (2.6)
Sex
Women
69,382 (48.9)
123,927 (55.1)
30,864 (54.9)
Men
66,163 (46.6)
94,258 (41.9)
23,713 (42.2)
Other or unknown
6,383 (4.5)
6,668 (3)
1,608 (2.8)
Race/Ethnicity§
American Indian or Alaska Native
342 (0.2)
426 (0.1)
104 (0.2)
Asian
7,451 (5.2)
18,043 (8.0)
8,341 (14.8)
Black or African American
12,319 (8.7)
13,359 (5.9)
2,632 (4.6)
Hispanic or Latino
42,973 (30.3)
79,198 (35.2)
14,023 (25.0)
Multiple race
494 (0.3)
968 (0.4)
210 (0.3)
Native Hawaiian or Other Pacific Islander
1,429 (1.0)
1,740 (0.7)
608 (1.0)
Other
18,720 (13.2)
32,552 (14.5)
6,808 (12.1)
White
20,529 (14.5)
34,108 (15.2)
12,504 (22.3)
Missing
37,671 (26.5)
44,459 (19.8)
10,955 (19.5)
Previously documented SARS-CoV-2 infection
12,360 (8.7)
22,153 (9.9)
3,246 (5.8)
Hospitalized
3,989 (2.8)
2,295 (1.0)
413 (0.7)
Admitted to an intensive care unit
641 (0.5)
276 (0.12)
47 (0.08)
Required mechanical ventilation
256 (0.2)
116 (0.05)
15 (0.03)
Died
485 (0.3)
172 (0.08)
40 (0.07)
Vaccine manufacturer¶
Johnson & Johnson
—
18,543 (8.2)
4,869 (8.7)
Moderna
—
82,435 (36.7)
19,742 (35.1)
Pfizer-BioNTech
—
123,875 (55.1)
31,574 (56.2)
Median interval between final vaccine dose and infection, days (IQR)**
—
241 (200–271)
49 (31–70)
Sequencing result available
7,087 (5.0)
9,663 (4.3)
1,296 (2.3)
Sequencing result
Delta
3,817 (53.9)
3,471 (35.9)
128 (9.9)
Omicron
3,248 (45.8)
6,180 (64.0)
1,164 (89.8)
Other
22 (0.3)
12 (0.1)
4 (0.3)
* Partially vaccinated persons were excluded from this analysis.
† A Pearson’s chi-square test was conducted for categorical variables and Kruskal-Wallis
test for medians; p<0.001.
§ Race and ethnicity were defined as mutually exclusive categories. Hispanic or Latino
includes all persons with ethnicity reported as “Hispanic or Latino” regardless of
reported race. “Other” Race/Ethnicity includes persons of multiple races, and persons
for whom reported race was “Other.” Missing values were included in statistical testing.
¶ The primary vaccine series was used to categorize persons by vaccine manufacturer
type regardless of which vaccine manufacturer was received for the booster dose.
** Infection date refers to the earliest of either the date of symptom onset, diagnosis,
death, report received, or specimen collection.
During the last week of Delta predominance (week ending December 11), age-adjusted
14-day cumulative incidence and hospitalization rates were highest among unvaccinated
persons (443.9 and 45.9 per 100,000 persons, respectively), and lower among fully
vaccinated persons with a booster (36.1 and 0.6, respectively) and fully vaccinated
persons without a booster (115.9 and 3.6, respectively). As Omicron became predominant,
age-adjusted incidence and hospitalization rates increased in all groups, irrespective
of vaccination status, compared with rates during the Delta predominant period (Figure
1). As of January 8, 2022, age-adjusted 14-day cumulative incidence and hospitalization
rates remained highest among unvaccinated persons (6,743.5 and 187.8 per 100,000,
respectively), and lowest among fully vaccinated persons with a booster (1,889.0 and
8.2, respectively) and fully vaccinated persons without a booster (3,355.5 and 35.4,
respectively).
FIGURE 1
Age-adjusted rolling 14-day SARS-CoV-2 cumulative incidence* (A) and hospitalization
rates (B), by vaccination status — Los Angeles County, California, November 7, 2021–January
8, 2022
* Rates were estimated using 2019 population estimates and standardized using the
year 2000 standard population.
The figure is a pair of line graphs indicating the age-adjusted rolling 14-day SARS-CoV-2
cumulative incidence in panel A and hospitalization rates in panel B, by vaccination
status in Los Angeles County, California during November 7, 2021–January 8, 2022.
Overall, during November 7, 2021–January 8, 2022, incidence and hospitalization rates
were highest among unvaccinated persons. During the last week of Delta predominance,
compared with fully vaccinated persons with a booster, incidence and hospitalization
rates among unvaccinated persons were 12.3 and 83.0 times higher, respectively (Figure
2), and compared with rates for fully vaccinated persons without a booster, incidence
and hospitalization rates among unvaccinated persons were 3.8 and 12.9 times higher,
respectively. As of January 8, 2022, during Omicron predominance, these rate ratios
were lower for both comparisons, with infection and hospitalization rates among unvaccinated
persons 3.6 times and 23.0 times, respectively, those in fully vaccinated persons
with a booster, and 2.0 and 5.3 times, respectively, those in fully vaccinated persons
without a booster.
FIGURE 2
Age-adjusted rolling 14-day SARS-CoV-2–associated incidence rate ratios* (A) and hospitalization
rate ratios (B), by vaccination status — Los Angeles County, California, November
7, 2021–January 8, 2022
* Rate ratios were estimated by comparing rates in unvaccinated persons with those
in vaccinated persons with and without a booster dose, using 2019 population estimates
and standardized using the year 2000 standard population.
The figure is a pair of line graphs indicating the age-adjusted rolling 14-day SARS-CoV-2–associated
incidence rate ratios in panel A and hospitalization rate ratios in panel B, by vaccination
status in Los Angeles County, California during November 7, 2021–January 8, 2022.
Discussion
During November 7, 2021–January 8, 2022, SARS-CoV-2 infections increased rapidly among
LAC adults with the largest increase occurring as Omicron displaced Delta as the predominant
circulating variant, leading to decreased incidence and hospitalization rate ratios
among unvaccinated persons relative to vaccinated persons with and without a booster.
Whereas incidence and hospitalization rates were higher during the Omicron-predominant
weeks compared with those during Delta predominance, rate ratios indicated continued
protection conferred by vaccine against severe disease, especially among those who
had received a booster, although reduced for Omicron compared with Delta. All incidence
and hospitalization rate ratios exceeded 1, regardless of predominant variant, indicating
that the risks were consistently highest for unvaccinated persons and that COVID-19
vaccines were protective against SARS-CoV-2 infection and COVID-19–associated hospitalization
among fully vaccinated persons, and most protective among those with a booster.
Although disease severity appears to be lower for Omicron, a rapid increase in infections
during Omicron predominance has resulted in a relatively substantial volume of hospitalizations
(
5
). The high volume of hospitalizations during a surge can compound the effects of
staffing shortages and staff member burnout, which puts a strain on the health care
sector. The rise in hospitalization rates in LAC was most pronounced among unvaccinated
persons, whereas hospitalization rates remained lower among those who were fully vaccinated,
and lowest among those who had received a booster. Being up to date with COVID-19
vaccinations is a critical component of reducing the strain on health care facilities.
The findings in this report are subject to at least five limitations. First, vaccination
data for persons who lived in LAC at the time of their laboratory-confirmed infection,
but who were vaccinated outside of California, were unavailable, leading to misclassification
of their vaccination status; if vaccinated persons without accessible records were
considered unvaccinated, the incidence in unvaccinated persons could be underestimated.
Some boosters might have been misclassified as first doses, and the persons receiving
these might have been incorrectly classified as partially vaccinated and excluded.
Second, aside from age adjustment, it was not possible to control for other factors
that are associated with vaccine coverage, such as sex and race/ethnicity. Differences
in vaccination and booster coverage by these characteristics, especially if proportionally
different from that of SARS-CoV-2 infections, could affect generalizability of these
results to LAC and other populations or jurisdictions. Third, the risks for SARS-CoV-2
infection are not equal for everyone; the likelihood of exposure might influence the
likelihood of COVID-19 vaccine acceptance and coverage. External risk factors related
to the possibility of infection and hospitalization, such as sample characteristics
and social determinants of health, are important to consider when interpreting these
findings. Fourth, COVID-19–associated hospitalizations were determined based on hospital
admission and SARS-CoV-2 test dates alone, potentially leading to the inclusion of
incidental positive SARS-CoV-2 test results in patients whose hospitalizations were
not caused by COVID-19. Finally, genomic sequencing data were available for only a
sample of SARS-CoV-2 specimens and not representative of all infections; however,
the variant predominance trends were consistent with what has been reported nationally
during these periods.
These findings align with those from recent studies, indicating that COVID-19 vaccination
protects against severe COVID-19 caused by SARS-CoV-2 variants, including Omicron
(
7
,
8
).*** Efforts to promote COVID-19 vaccination and boosters are critical to preventing
COVID-19–associated hospitalizations and severe outcomes. Ongoing COVID-19 surveillance
with data linkages to vaccination and SARS-CoV-2 variant genomic sequencing data are
critical for monitoring vaccine effectiveness and increased protection from boosters,
particularly during the Omicron predominant period.
Summary
What is already known about this topic?
COVID-19 vaccines are highly effective against severe SARS-CoV-2–associated outcomes,
including those caused by the Delta variant.
What is added by this report?
As of January 8, 2022, during Omicron predominance, COVID-19 incidence and hospitalization
rates in Los Angeles County among unvaccinated persons were 3.6 and 23.0 times, respectively,
those of fully vaccinated persons with a booster, and 2.0 and 5.3 times, respectively,
those among fully vaccinated persons without a booster. During both Delta and Omicron
predominance, incidence and hospitalization rates were highest among unvaccinated
persons and lowest among vaccinated persons with a booster.
What are the implications for public health practice?
Being up to date with COVID-19 vaccination is critical to protecting against SARS-CoV-2
infection and hospitalization.