Frequently Asked Questions (FAQs)

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The Napa County Epidemiology team developed this page to answer some of the most common questions about COVID-19 data and the COVID-19 data dashboards.  
For additional information please contact the Public Health Main Line:
Phone: 707-253-4270 (Monday – Friday, from 8 a.m. to 5 p.m.)

Q1: What is a case?
A1: A case is a person identified as having a particular disease or condition.

Q2: What is a case definition?
A2: A case definition is a set of uniform criteria used to define a disease for public health surveillance. Case definitions allow public health officials to count cases consistently across reporting jurisdictions. Case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs. A case definition can change over time, due to continual advancement in the science of a disease and changes to surveillance approaches during outbreaks of the disease. 

Q3: What is case surveillance?
A3: Case surveillance is a key part of public health practice. Case surveillance helps public health officials understand diseases and how they spread. It also helps determine suitable actions to control outbreaks. Case surveillance occurs each time Napa County, the California Department of Public Health (CDPH), or the Centers for Disease Control and Prevention (CDC) collect information about a case that poses a serious health threat to the community. These diseases and conditions include infectious diseases such as COVID-19, foodborne outbreaks such as E. coli, and noninfectious conditions such as lead poisoning.

Q4: How does Napa County get case information?
A4: Some diseases and conditions, like COVID-19, are required to be reported by healthcare facilities, like hospitals and skilled nursing facilities, healthcare providers, like physicians, dentists, and veterinarians, laboratories, and schools, including childcare centers, to local and/or state health departments. Laws and regulations, including Title 17, California Code of Regulations (CCR) 2500, 2593, 2641.5-2643.20, and 2800-2812 Reportable Diseases and Conditions, specify which diseases and conditions must be reported.

Q5: How does Napa County use case information?
A5: Napa County works with healthcare facilities, healthcare providers, laboratories, schools, and other community partners to get the information needed to monitor, control, and prevent reportable diseases and conditions in Napa County. Some diseases and conditions, like COVID-19, are also monitored by CDPH and CDC to help protect community health at the local, state, and national level.

Q6: What information does case surveillance collect?
A6: Case surveillance follows standard case definitions and collects information that public health officials can use to understand where diseases are occurring, how they can be prevented, and which populations are most affected.   

Q7: What is the case definition for COVID-19?
A7: A case of COVID-19 is defined as a person who meets at least one of the following criteria:
  • Laboratory Criteria – Laboratory evidence using a method approved or authorized by the United States Food and Drug Administration (FDA) or designated authority:
    • Confirmatory laboratory evidence:
      • Detection of SARS-CoV-2 ribonucleic acid (RNA) in a clinical or post-mortem specimen using a diagnostic molecular amplification test performed by a Clinical Laboratory Improvement Amendments (CLIA)-certified provider, OR
      • Detection of SARS-CoV-2 RNA in a clinical or post-mortem specimen by genomic sequencing.
    • Presumptive laboratory evidence:
      • Detection of SARS-CoV-2 specific antigen in a clinical or post-mortem specimen using a diagnostic test performed by a CLIA-certified provider.
    • Supportive laboratory evidence:
      • Detection of SARS-CoV-2 specific antigen by immunocytochemistry, OR
      • Detection of SARS-CoV-2 RNA or specific antigen using a test performed without CLIA oversight.
NOTE: The terms confirmatory, presumptive, and supportive are categorical labels used here to standardize case classifications for public health surveillance. The terms should not be used to interpret the effectiveness or validity of any laboratory test methodology.

Q8: If a person tests positive more than once in a row, do they get counted as a case more than one time?
A8: A person is only counted as a case once when repeat testing is done with a 90-day period.

Q9: How can you distinguish a new case from an existing case?
A9: The following criteria can be used to distinguish a new case from an existing case:
  • A person was most recently counted as a confirmed or probable case with onset date (if available) or first positive specimen collection date for that classification is more than 90 days prior. 
  • SARS-CoV-2 sequencing results from the new positive specimen and a positive specimen from the most recent previous case demonstrate a different lineage.  
  • A person was previously reported but not counted as a confirmed or probable case (i.e. suspect), but now meets the criteria for a confirmed or probable case.

Q10: How do you count a COVID-19 hospitalized case?
A10: A COVID-19 hospitalized case is a person who,
  • has tested positive for COVID-19 by polymerase chain reaction (PCR)
  • is hospitalized due to symptoms associated with COVID-19 infection.

Q11: How do you count a COVID-19 death case?
A11: Based on guidance from the Council of State and Territorial Epidemiologists (CSTE) and the CDC, the criteria that Napa County is using to identify death cases associated with COVID-19 is as follows:
  • The death certificate indicates COVID-19 or an equivalent term as an immediate, underlying, or contributing cause of death. This includes cases when COVID-19 is listed in the “Other Conditions” column on the death certificate.
  • A case investigation for a confirmed, probable, or suspected SARS-CoV-2 infection case determined that COVID-19 was the cause of death or contributed to the death despite not being listed as a cause of death in the death certificate. Cause of death can include acute respiratory failure, pneumonia, and variations of respiratory ailments commonly associated with COVID-19 disease.

Q12: What is wastewater surveillance and how does Napa County use wastewater data?
A12: Wastewater data is used to supplement other public health data to keep the community informed about the local COVID-19 situation so they can decide how to best respond. People infected with COVID-19 shed the virus in their feces whether or not they experience symptoms. Wastewater, or sewage, includes water from household and building use, such as toilets, showers, and sinks, that can contain human fecal waste. As the wastewater flows to the water treatment facility, samples are collected and sent to laboratories to test for the presence of the COVID-19 virus. 
Wastewater surveillance can include testing for other viruses and infections in addition to COVID-19, such as influenza, RSV, norovirus, and MPOX. Napa County uses the data collected by wastewater surveillance to monitor for the presence of infections in near real time and track local disease trends.

Q13: What reporting changes did Napa County make after the end of the Public Health Emergency?
A13: Monitoring the impact of COVID-19 as well as the effectiveness of prevention and control strategies remains a public health priority. When the COVID-19 Public Health Emergency ended on May 11, 2023, some data metrics changed in frequency, source, or availability. This is in part because CDC, CDPH, and Napa County’s ability to collect and share certain types of data changed. For example, COVID-19 case data has stopped because it has become unreliable, mainly due to changes in PCR testing, such as individuals testing at home using non-reportable antigen tests. More details explaining COVID-19 surveillance after the expiration of the Public Health Emergency can be found here.

Q14: What are variants of COVID-19?
A14: Viruses are constantly changing, including the virus that causes COVID-19. These changes are caused by random mutations when the virus replicates, and over time, branch off into new strains of the virus. These new strains are called variants.  New variants will continue to emerge. The Centers for Disease Control and Prevention (CDC) tracks and classifies circulating variants. Detailed and up to date information on COVID-19 variants can be found in this link

Q15: What is the difference between being fully vaccinated and being up to date?
A15: A person is considered fully vaccinated when they have completed only a primary series. A person is considered up to date when they have received ALL COVID-19 vaccine dose(s) recommended for them by the CDC. Being fully vaccinated is not the same as having the best protection that staying up to date provides because you can be fully vaccinated without completing all vaccine doses recommended for you.

Q16:  What is a rate and how is it interpreted?
A16:  A rate measures how frequently a disease or health event occurs within a population over a specific period of time. A rate is interpreted as the number of people with the disease per the total population within a specific time. An example of a rate is 80 new cases of COVID-19 per 10,000 Napa County residents per month. Rates allow comparisons of cases among places that have different population sizes. Napa County, compared to other counties in the Bay Area, has a smaller population so an equal absolute number of cases in our county has more impact compared to the same number in a county with twice as many residents. 

Q17: How do we track vaccination in our community?
A17:  The California Immunization Registry contains records for everyone who has received a COVID-19 vaccine. This information can be matched with case, hospitalized, and deceased records in the California Department of Public Health’s disease surveillance system to track impact of vaccination on these metrics. We can also use this data to compare to the total population size or size of subcategories like age and ethnicity and calculate rates and coverage percentages. Cases that do not have a vaccine record in the database would be considered unvaccinated in our calculations.

For more information on COVID-19, including state and national level data and guidance, please visit the below COVID-19 home pages.