COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis

California and New York, May-November 2021

Tomás M. León, PhD; Vajeera Dorabawila, PhD; Lauren Nelson, MPH; Emily Lutterloh, MD; Ursula E. Bauer, PhD; Bryon Backenson, MPH; Mary T. Bassett, MD; Hannah Henry, MPH; Brooke Bregman, MPH; Claire M. Midgley, PhD; Jennifer F. Myers, MPH; Ian D. Plumb, MBBS; Heather E. Reese, PhD; Rui Zhao, MPH; Melissa Briggs-Hagen, MD; Dina Hoefer, PhD; James P. Watt, MD; Benjamin J. Silk, PhD; Seema Jain, MD; Eli S. Rosenberg, PhD

Disclosures

Morbidity and Mortality Weekly Report. 2022;71(4):125-131.

Abstract and Introduction

Introduction

By November 30, 2021, approximately 130,781 COVID-19–associated deaths, one in six of all U.S. deaths from COVID-19, had occurred in California and New York.* COVID-19 vaccination protects against infection with SARS-CoV-2 (the virus that causes COVID-19), associated severe illness, and death;^[1,2] among those who survive, previous SARS-CoV-2 infection also confers protection against severe outcomes in the event of reinfection.^[3,4]The relative magnitude and duration of infection- and vaccine-derived protection, alone and together, can guide public health planning and epidemic forecasting. To examine the impact of primary COVID-19 vaccination and previous SARS-CoV-2 infection on COVID-19 incidence and hospitalization rates, statewide testing, surveillance, and COVID-19 immunization data from California and New York (which account for 18% of the U.S. population) were analyzed. Four cohorts of adults aged ≥18 years were considered: persons who were 1) unvaccinated with no previous laboratory-confirmed COVID-19 diagnosis, 2) vaccinated (14 days after completion of a primary COVID-19 vaccination series) with no previous COVID-19 diagnosis, 3) unvaccinated with a previous COVID-19 diagnosis, and 4) vaccinated with a previous COVID-19 diagnosis. Age-adjusted hazard rates of incident laboratory-confirmed COVID-19 cases in both states were compared among cohorts, and in California, hospitalizations during May 30–November 20, 2021, were also compared. During the study period, COVID-19 incidence in both states was highest among unvaccinated persons without a previous COVID-19 diagnosis compared with that among the other three groups. During the week beginning May 30, 2021, compared with COVID-19 case rates among unvaccinated persons without a previous COVID-19 diagnosis, COVID-19 case rates were 19.9-fold (California) and 18.4-fold (New York) lower among vaccinated persons without a previous diagnosis; 7.2-fold (California) and 9.9-fold lower (New York) among unvaccinated persons with a previous COVID-19 diagnosis; and 9.6-fold (California) and 8.5-fold lower (New York) among vaccinated persons with a previous COVID-19 diagnosis. During the same period, compared with hospitalization rates among unvaccinated persons without a previous COVID-19 diagnosis, hospitalization rates in California followed a similar pattern. These relationships changed after the SARS-CoV-2 Delta variant became predominant (i.e., accounted for >50% of sequenced isolates) in late June and July. By the week beginning October 3, compared with COVID-19 cases rates among unvaccinated persons without a previous COVID-19 diagnosis, case rates among vaccinated persons without a previous COVID-19 diagnosis were 6.2-fold (California) and 4.5-fold (New York) lower; rates were substantially lower among both groups with previous COVID-19 diagnoses, including 29.0-fold (California) and 14.7-fold lower (New York) among unvaccinated persons with a previous diagnosis, and 32.5-fold (California) and 19.8-fold lower (New York) among vaccinated persons with a previous diagnosis of COVID-19. During the same period, compared with hospitalization rates among unvaccinated persons without a previous COVID-19 diagnosis, hospitalization rates in California followed a similar pattern. These results demonstrate that vaccination protects against COVID-19 and related hospitalization, and that surviving a previous infection protects against a reinfection and related hospitalization. Importantly, infection-derived protection was higher after the Delta variant became predominant, a time when vaccine-induced immunity for many persons declined because of immune evasion and immunologic waning.

Abstract and Introduction
Discussion
References
Sidebar

*https://covid.cdc.gov/covid-data-tracker/#cases_deathsper100klast7days
^†Statewide immunization databases in California are the California Immunization Registry, Regional Immunization Data Exchange, and San Diego Immunization Registry; the laboratory system is the California COVID Reporting System (CCRS). In New York, immunization information systems include Citywide Immunization Registry and the New York State Immunization Information System; the laboratory system is the Electronic Clinical Laboratory Reporting System (ECLRS). California data were matched between the immunization and case registries using a probabilistic algorithm with exact match for zip code and date of birth and fuzzy match on first name and last name. New York data were matched to the ECLRS with the use of a deterministic algorithm based on first name, last name, and date of birth. In California, person-level hospitalization data from CCRS and supplementary hospitalization reports were used to identify COVID-19–associated hospitalizations.
^§For both classification into cohorts of persons with previous COVID-19 diagnoses and for measuring incident cases, laboratory-confirmed infection was defined as the receipt of a new positive SARS-CoV-2 nucleic acid amplification test (NAAT) or antigen test (both for New York and NAAT only for California) result, but not within 90 days of a previous positive result.
^¶Fully vaccinated with the primary vaccination series is defined as receipt of a second dose of an mRNA COVID-19 vaccine (Pfizer-BioNTech or Moderna) or 1 dose of the Janssen (Johnson & Johnson) vaccine ≥14 days before May 30, 2021.
**Because of the timing of full vaccination, the cohort definitions, and analysis timeframe, this cohort consisted nearly exclusively of persons who had previously received a laboratory-confirmed diagnosis of COVID-19 and later were fully vaccinated (California: 99.9%, New York: 99.7%), as opposed to the reverse order.
^††Whereas vaccinated cohorts were directly observed in the immunization information system databases, unvaccinated persons without a previous COVID-19 diagnosis were defined using U.S. Census population estimates minus the number of persons partially or fully vaccinated by December 11, 2021, and unvaccinated persons with a previous laboratory-confirmed infection before May 30, 2021. In California, the California Department of Finance population estimates were used for 2020, and the 2018 CDC National Center for Health Statistics Bridged Race file for U.S. Census population estimates were used in New York, consistent with other COVID-19 surveillance reporting.
^§§In California, a person-level match was performed to exclude deaths in each cohort before May 30, 2021. In New York, COVID-19 deaths were removed in aggregate from the starting number of unvaccinated persons with a previous COVID-19 diagnosis on May 30, 2021.
^¶¶ https://www.cdc.gov/nchs/data/statnt/statnt20.pdf
***https://covid.cdc.gov/covid-data-tracker/#variant-proportions

Table 1. Cohort sizes and cohort-specific incident laboratory-confirmed COVID-19 cases in California (N = 752,781) and New York (N = 355,819) and hospitalizations in California (N = 56,177) — May 30–November 20, 2021
State/Vaccination and diagnosis status*^,†	No. of persons in each cohort (%)	Incident laboratory-confirmed COVID-19 cases			Incident COVID-19 hospitalizations**
State/Vaccination and diagnosis status*^,†	No. of persons in each cohort (%)	No. (cumulative incidence)^§,¶	Median (IQR) interval from vaccination to positive test, days	Median (IQR) interval from previous diagnosis to positive test, days	No. (cumulative incidence)^§,¶
California
Vaccinated
Previous COVID-19 diagnosis	968,167 (4.5)	3,471 (3.6)	138 (95–181)	262 (218–322)	273 (0.3)
No previous diagnosis	15,484,235 (71.2)	240,045 (15.5)	150 (112–189)	NA	10,737 (0.7)
Unvaccinated
Previous COVID-19 diagnosis	1,370,782 (6.3)	6,805 (5.0)	NA	277 (229–356)	378 (0.3)
No previous diagnosis	3,911,146 (18.0)	502,460 (128.5)	NA	NA	44,789 (11.5)
New York
Vaccinated
Previous COVID-19 diagnosis	485,649 (4.5)	2,355 (4.9)	162 (118–201)	276 (227–348)	NA
No previous diagnosis	7,809,968 (72.2)	142,388 (18.2)	171 (133–203)	NA	NA
Unvaccinated
Previous COVID-19 diagnosis	527,140 (4.9)	3,250 (6.2)	NA	295 (242–427)	NA
No previous diagnosis	1,993,709 (18.4)	207,826 (104.2)	NA	NA	NA

Abbreviations: NA = not applicable; NAAT = nucleic acid amplification test.
*Statewide immunization databases in California are the California Immunization Registry, Regional Immunization Data Exchange, and San Diego Immunization Registry, and the laboratory system is the California COVID Reporting System; in New York, Immunization Information Systems include Citywide Immunization Registry and the New York State Immunization Information System; the laboratory system is the Electronic Clinical Laboratory Reporting System. California data were matched between the immunization and case registries using a probabilistic algorithm with exact match for zip code and date of birth and fuzzy match on first name and last name. New York data were matched to the Electronic Clinical Laboratory Reporting System with the use of a deterministic algorithm based on first name, last name, and date of birth. In California, person-level hospitalization data from the California COVID Reporting System and supplemental hospitalization reports were used to identify COVID-19-associated hospitalizations.
^†For both classification into cohorts of persons with previous COVID-19 diagnoses and for measuring incident cases, laboratory-confirmed infection was defined as the receipt of a new positive SARS-CoV-2 NAAT or antigen test (both for New York and NAAT only for California) result, but not within 90 days of a previous positive result. Fully vaccinated is defined as having received a second dose of an mRNA COVID-19 vaccine (Pfizer-BioNTech or Moderna) or 1 dose of the Janssen (Johnson & Johnson) vaccine ≥14 days before May 30, 2021. Whereas vaccinated cohorts were directly observed in the immunization information system databases, unvaccinated persons without a previous COVID-19 diagnosis were defined using U.S. Census population estimates minus persons partially or fully vaccinated by December 11, 2021, and unvaccinated persons with a previous laboratory-confirmed infection before May 30, 2021. In California, the California Department of Finance population estimates were used for 2020, and the 2018 CDC National Center for Health Statistics Bridged Race file for census population estimates were used in New York, consistent with other COVID-19 surveillance reporting.
^§Cumulative cases per 1,000 persons.
^¶These summaries of cumulative incidence are estimated across a period of variability in the epidemic for all cohorts.
**Hospitalization data for New York are not included in this analysis.

Table 2. Hazard ratios for incident laboratory-confirmed COVID-19 cases — New York and California and hospitalizations* — California, May 30– November 20, 2021
State and date range	Hazard ratio (95% CI)^†
	Unvaccinated, no previous COVID-19 diagnosis versus			Vaccinated, no previous COVID-19 diagnosis versus
	Vaccinated, no previous COVID-19 diagnosis	Unvaccinated, previous COVID-19 diagnosis	Vaccinated, previous COVID-19 diagnosis	Unvaccinated, previous COVID-19 diagnosis	Vaccinated, previous COVID-19 diagnosis
Cases, California
May 30–Jun 5	20.9 (18.9–22.9)	8.2 (6.6–9.9)	10.6 (8.1–13.2)	0.4 (0.3–0.5)	0.5 (0.4–0.6)
Jun 6–12	17.9 (16.2–19.5)	8.6 (6.8–10.4)	10.5 (7.9–13.0)	0.5 (0.4–0.6)	0.6 (0.4–0.7)
Jun 13–19	16.0 (14.7–17.4)	10.8 (8.5–13.2)	10.6 (8.2–13.1)	0.7 (0.5–0.8)	0.7 (0.5–0.8)
Jun 20–26	12.3 (11.4–13.1)	14.5 (11.2–17.8)	17.3 (12.8–21.8)	1.2 (0.9–1.5)	1.4 (1.0–1.8)
Jun 27–Jul 3	9.7 (9.2–10.2)	16.6 (13.5–19.7)	20.9 (16.0–25.8)	1.7 (1.4–2.0)	2.2 (1.6–2.7)
Jul 4–10	8.7 (8.4–9.0)	24.0 (20.1–28.0)	29.3 (23.1–35.6)	2.8 (2.3–3.2)	3.4 (2.6–4.1)
Jul 11–17	7.8 (7.5–8.0)	29.0 (25.0–32.9)	33.4 (27.3–39.4)	3.7 (3.2–4.2)	4.3 (3.5–5.1)
Jul 18–24	7.4 (7.2–7.6)	31.8 (28.1–35.6)	35.2 (29.8–40.6)	4.3 (3.8–4.8)	4.7 (4.0–5.5)
Jul 25–31	7.5 (7.4–7.7)	26.5 (24.1–29.0)	38.6 (33.3–43.9)	3.5 (3.2–3.8)	5.1 (4.4–5.8)
Aug 1–7	7.8 (7.6–7.9)	32.6 (29.5–35.6)	42.2 (36.7–47.7)	4.2 (3.8–4.6)	5.4 (4.7–6.1)
Aug 8–14	8.1 (7.9–8.2)	33.4 (30.4–36.5)	43.1 (37.6–48.6)	4.1 (3.8–4.5)	5.3 (4.7–6.0)
Aug 15–21	8.4 (8.3–8.6)	31.3 (28.5–34.1)	42.0 (36.7–47.3)	3.7 (3.4–4.0)	5.0 (4.3–5.6)
Aug 22–28	8.4 (8.3–8.6)	31.3 (28.4–34.3)	41.0 (35.5–46.5)	3.7 (3.4–4.1)	4.9 (4.2–5.5)
Aug 29–Sep 4	8.5 (8.3–8.6)	31.2 (28.1–34.3)	42.0 (36.1–48.0)	3.7 (3.3–4.1)	5.0 (4.3–5.7)
Sep 5–11	8.3 (8.1–8.5)	35.0 (31.0–39.0)	48.0 (40.2–55.9)	4.2 (3.7–4.7)	5.8 (4.8–6.7)
Sep 12–18	8.4 (8.2–8.6)	33.8 (29.9–37.8)	48.0 (39.8–56.2)	4.0 (3.6–4.5)	5.7 (4.7–6.7)
Sep 19–25	8.0 (7.8–8.2)	27.0 (23.8–30.1)	37.8 (31.5–44.1)	3.4 (3.0–3.8)	4.7 (4.0–5.5)
Sep 26–Oct 2	7.7 (7.5–7.9)	28.6 (24.9–32.2)	34.8 (28.9–40.7)	3.7 (3.2–4.2)	4.5 (3.7–5.3)
Oct 3–9	7.2 (7.0–7.4)	30.0 (26.0–34.1)	33.5 (28.5–38.6)	4.1 (3.6–4.7)	4.6 (3.9–5.3)
Oct 10–16	7.2 (7.0–7.4)	31.2 (26.8–35.7)	33.9 (27.8–40.0)	4.3 (3.7–5.0)	4.7 (3.9–5.5)
Oct 17–23	7.1 (7.0–7.3)	31.9 (27.6–36.1)	40.7 (33.3–48.1)	4.5 (3.9–5.0)	5.7 (4.7–6.7)
Oct 24–30	7.1 (6.9–7.3)	26.6 (23.3–29.9)	40.1 (32.9–47.3)	3.7 (3.3–4.2)	5.6 (4.6–6.6)
Oct 31–Nov 6	6.8 (6.6–7.0)	33.1 (28.7–37.6)	37.9 (31.0–44.7)	4.9 (4.2–5.5)	5.5 (4.5–6.6)
Nov 7–13	7.1 (6.9–7.3)	30.6 (26.3–35.0)	41.2 (33.0–49.5)	4.3 (3.7–4.9)	5.8 (4.6–7.0)
Nov 14–20	7.3 (7.0–7.5)	25.4 (21.4–29.3)	32.5 (25.5–39.5)	3.5 (2.9–4.0)	4.5 (3.5–5.5)
Cases, New York
May 30–Jun 5	19.4 (16.9–21.8)	10.9 (7.5–14.3)	9.5 (6.7–12.4)	0.6 (0.4–0.7)	0.5 (0.3–0.7)
Jun 6–12	15.2 (13.2–17.2)	8.0 (5.5–10.6)	10.4 (6.6–14.3)	0.5 (0.4–0.7)	0.7 (0.4–0.9)
Jun 13–19	12.8 (11–14.5)	8.2 (5.3–11.2)	5.4 (3.7–7.0)	0.6 (0.4–0.9)	0.4 (0.3–0.6)
Jun 20–26	10.1 (8.8–11.4)	7.9 (5.1–10.7)	6.0 (4.0–8.0)	0.8 (0.5–1.1)	0.6 (0.4–0.8)
Jun 27–Jul 3	7.3 (6.5–8.1)	8.8 (5.8–11.8)	11.2 (6.7–15.7)	1.2 (0.8–1.6)	1.5 (0.9–2.2)
Jul 4–10	6.1 (5.6–6.7)	17.8 (10.6–25.0)	11.5 (7.5–15.6)	2.9 (1.7–4.1)	1.9 (1.2–2.6)
Jul 11–17	4.5 (4.2–4.8)	11.7 (8.5–15.0)	14.7 (9.9–19.6)	2.6 (1.9–3.3)	3.2 (2.2–4.3)
Jul 18–24	4.7 (4.5–5.0)	21.7 (15.6–27.8)	14.1 (10.5–17.7)	4.6 (3.3–5.9)	3.0 (2.2–3.8)
Jul 25–31	5.1 (4.9–5.3)	16.1 (13.1–19.2)	18.3 (14.1–22.6)	3.2 (2.6–3.8)	3.6 (2.8–4.4)
Aug 1–7	5.3 (5.2–5.5)	19.2 (15.9–22.6)	18.3 (14.7–21.9)	3.6 (3.0–4.2)	3.4 (2.7–4.1)
Aug 8–14	5.3 (5.2–5.5)	16.2 (13.8–18.6)	19.2 (15.6–22.7)	3.0 (2.6–3.5)	3.6 (2.9–4.3)
Aug 15–21	5.5 (5.3–5.7)	19.5 (16.5–22.6)	22.7 (18.4–26.9)	3.6 (3.0–4.1)	4.1 (3.4–4.9)
Aug 22–28	5.4 (5.2–5.6)	19.2 (16.4–22.1)	26.5 (21.2–31.8)	3.6 (3.0–4.1)	4.9 (3.9–5.9)
Aug 29–Sep 4	5.5 (5.3–5.6)	17.9 (15.3–20.5)	20.9 (17.2–24.6)	3.3 (2.8–3.8)	3.8 (3.1–4.5)
Sep 5–11	5.4 (5.2–5.5)	18.9 (16.1–21.6)	22.3 (18.3–26.4)	3.5 (3.0–4.0)	4.2 (3.4–4.9)
Sep 12–18	5.8 (5.6–5.9)	15.0 (13.1–16.9)	23.2 (19.1–27.4)	2.6 (2.3–2.9)	4.0 (3.3–4.8)
Sep 19–25	5.6 (5.4–5.7)	15.4 (13.3–17.5)	23.8 (19.3–28.3)	2.8 (2.4–3.1)	4.3 (3.5–5.1)
Sep 26–Oct 2	5.4 (5.2–5.5)	18.4 (15.5–21.2)	24.2 (19.3–29.1)	3.4 (2.9–4.0)	4.5 (3.6–5.4)
Oct 3–9	5.5 (5.3–5.7)	15.7 (13.6–17.9)	20.8 (17.2–24.5)	2.9 (2.5–3.3)	3.8 (3.1–4.4)
Oct 10–16	5.5 (5.3–5.6)	17.2 (14.7–19.8)	25.9 (20.6–31.1)	3.2 (2.7–3.6)	4.7 (3.8–5.7)
Oct 17–23	5.4 (5.2–5.6)	18.9 (15.7–22.1)	27.6 (21.2–34.0)	3.5 (2.9–4.1)	5.1 (3.9–6.3)
Oct 24–30	5.2 (5.0–5.4)	21.0 (17.2–24.7)	25.9 (20.2–31.6)	4.0 (3.3–4.7)	5.0 (3.9–6.1)
Oct 31–Nov 6	4.8 (4.6–4.9)	17.3 (14.7–20.0)	20.1 (16.3–23.8)	3.6 (3.1–4.2)	4.2 (3.4–5.0)
Nov 7–13	4.8 (4.7–4.9)	23.9 (20.1–27.6)	24.5 (20.1–28.9)	5.0 (4.2–5.8)	5.1 (4.2–6.1)
Nov 14–20	4.8 (4.6–4.9)	22.6 (19.4–25.7)	23.0 (19.3–26.6)	4.7 (4.1–5.4)	4.8 (4.1–5.6)
Hospitalizations, California
May 30–Jun 12	29.8 (23.5–36.1)	3.7 (2.5–5.0)	7.2 (4.2–10.1)	0.1 (0.1–0.2)	0.2 (0.1–0.3)
Jun 13–26	28.7 (23.4–34.0)	7.0 (4.3–9.7)	8.1 (5.0–11.3)	0.2 (0.1–0.3)	0.3 (0.2–0.4)
Jun 27–10	30.1 (26.1–34.0)	16.4 (10.0–22.8)	16.0 (10.0–22.1)	0.5 (0.3–0.8)	0.5 (0.3–0.7)
Jul 11–24	25.8 (23.7–28.0)	45.0 (27.6–62.4)	41.5 (25.2–57.8)	1.7 (1.1–2.4)	1.6 (1.0–2.2)
Jul 25–Aug 7	28.8 (27.1–30.6)	41.7 (29.2–54.1)	72.9 (44.4–101.4)	1.4 (1.0–1.9)	2.5 (1.5–3.5)
Aug 8–21	29.7 (28.0–31.4)	49.0 (35.0–62.9)	64.0 (43.0–85.1)	1.6 (1.2–2.1)	2.2 (1.4–2.9)
Aug 22–Sep 4	29.1 (27.4–30.8)	62.4 (41.4–83.3)	63.9 (42.2–85.5)	2.1 (1.4–2.9)	2.2 (1.4–2.9)
Sep 5–18	26.3 (24.6–28.1)	74.4 (40.9–107.9)	96.4 (48.3–144.4)	2.8 (1.5–4.1)	3.7 (1.8–5.5)
Sep 19–Oct 2	25.0 (23.1–26.9)	61.9 (34.5–89.3)	99.4 (43.8–155.0)	2.5 (1.4–3.6)	4.0 (1.7–6.2)
Oct 3–16	20.8 (19.2–22.4)	56.3 (28.3–84.3)	58.5 (30.2–86.8)	2.7 (1.4–4.1)	2.8 (1.4–4.2)
Oct 17–30	21.5 (19.9–23.0)	56.5 (31.5–81.5)	92.1 (39.1–145.1)	2.6 (1.5–3.8)	4.3 (1.8–6.8)
Oct 31–Nov 13	22.7 (20.8–24.6)	70.7 (32.0–109.4)	86.1 (34.2–138.1)	3.1 (1.4–4.8)	3.8 (1.5–6.1)

*Life tables estimated at 7-day intervals for cases and 14-day intervals for hospitalizations.
^†Hazard ratios and 95% CIs reported in this table differ numerically from presentation of corresponding results in the text as "X-fold lower" rates (i.e., a hazard rate of 1.0 is zero-fold lower). For example, a hazard ratio of 20.9 (95% CI = 18.9–22.9) for those "Unvaccinated–no previous COVID-19 diagnosis" versus "Vaccinated, no previous COVID-19 diagnosis" is equivalent to a 19.9-fold lower (95% CI = 17.9–21.9) rate for those "Vaccinated, no previous COVID-19 diagnosis" relative to those "Unvaccinated, no previous COVID-19 diagnosis."

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COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis

California and New York, May-November 2021

Abstract and Introduction

Introduction

Tables

Tables

References

Authors and Disclosures

Authors and Disclosures

Sidebar

Sidebar

Summary

Figure.

Figure.

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