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.)
Email: CoronaVirus@countyofnapa.org
Q1: What is a case and how are they counted?
A1: Epidemiologists count cases of disease, calculate rates, and compare the rates over time or between different groups of people. A case definition is a set of criteria used to determine who has a case of a specific disease. This allows epidemiologists and other public health officials to be consistent when classifying and counting cases across all jurisdictions during public health surveillance.
- Example: The case definition for COVID-19 is someone who meets at least one of the following criteria:
- Clinical criteria:
- Has an acute onset or worsening of at least two signs or symptoms: fever, chills, rigors, myalgia, headache, sore throat, nausea or vomiting, diarrhea, fatigue, congestion or runny nose, OR
- Has an acute onset or worsening of one of the following signs and symptoms: cough, shortness of breath, taste disorder, difficulty breathing, olfactory disorder, confusion or change in mental status, persistent pain or pressure in the chest, pale, gray, or blue-colored skin, lips, or nail beds, depending on skin tone, inability to wake or stay awake, OR
- Has severe respiratory illness with either clinical or radiographic evidence of pneumonia or acute respiratory distress syndrome (ARDS).
- Lab criteria:
- Has FDA or designated authority lab evidence.
- Epidemiologic linkage:
- Has one or more of the following exposures in the last 14 days: close contact with a confirmed or probable case of COVID-19 OR is a member of an exposed risk cohort.
Once a case definition is created, an epidemiologist can identify cases and collect information.
Q2: If a person tests positive more than once in a row, do they get counted as a case more than one time?
A2: A person is only counted as a case once, when repeat testing is done.
Q3: What is a reinfection and when is a reinfection counted as a new case?
A3: A reinfection is when a person had the disease, recovered, and then, later, got the disease again.
If a person has a reinfection, they are counted as a new case when:
- their sequencing results from a new positive specimen and a positive specimen from the most recent previous case demonstrates a different lineage, OR
- a person was counted as a confirmed or probable case and the onset date or the first positive test result date for that classification was more than 90 days prior, OR
- a person was previously reported but not counted as a confirmed or probable case, but now meets the criteria for a confirmed or probable case.
Q4: If a person tests negative after they test positive, do they still need to isolate?
A4: If a symptomatic person tests negative after they test positive, they no longer need to isolate if:
- it has been more than 10 days since symptoms began, AND
- they have been without a fever for more than 24 hours without the use of fever-reducing medications, AND
- other symptoms have improved.
If an asymptomatic person tests negative after they test positive, they can stop isolation 10 days after they first tested positive.
Q5: Does the county only investigate and report hospitalized and fatal breakthrough cases following the CDC protocol?
A5: No, the county does not follow the CDC protocol, which only reports hospitalized and fatal breakthrough cases. The county follows CDPH guidance that investigates all breakthrough cases, in addition to hospitalized and fatal breakthrough cases.
Q6: How do you count a COVID-19 hospitalized case?
A6: A person who has tested positive for COVID-19 by polymerase chain reaction (PCR) and is hospitalized due to symptoms associated to COVID-19 infection is counted as a COVID-19 hospitalized case.
Q7: How do you count a COVID-19 death case?
A7: Based on guidance from the Council of State and Territorial Epidemiologists (CSTE) and CDC, the criteria that the Napa County Public Health is using to identify death cases associated with COVID-19 is as follows:
1. The case meets the case definition for COVID-19 AND at least ONE of the following:
- COVID-19 was the determined cause of death or contributor to death by a case investigation,
- COVID-19 was indicated as one of the causes of death on the death certificate,
- The death occurred within 30 days of when the laboratory test was performed and was due to natural causes, such as age or disease, as opposed to an accident or violence.
- Example: On the death certificate, the Manner of Death is coded as “natural”. OR
2. The case meets the case definition for probable COVID-19 AND COVID-19 was the determined cause of death or contributor to death by a case investigation. OR
3. The case meets the case definition for probable COVID-19 based on laboratory evidence AND death occurred within 30 days of when the laboratory test was performed and was due to natural causes, such as age or disease, as opposed to an accident or violence. OR
4. The case meets the case definition for probable COVID-19 based on epidemiologic linkage and meeting the clinical criteria AND death occurred within 30 days of symptom onset and was due to natural causes, such as age or disease, as opposed to an accident or violence. OR
5. The case meets the case definition for probable COVID-19 based only on vital records criteria AND there is no laboratory evidence.
Q8: How do you calculate the percent of Napa County residents that are hospitalized?
A8: The percent of Napa County residents that are hospitalized is calculated by the number of Napa County residents who are hospitalized and have a positive COVID-19 lab test divided by the total number of Napa County residents.
Q9: What is a rate and how is it interpreted?
A9: 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.
- Example: 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 amount in a county with twice as many residents.
Q10: What does it mean to be Unvaccinated, Fully-Vaccinated, and Partially-Vaccinated?
A10: An unvaccinated individual has never received any doses of any type of COVID-19 vaccines.
A fully-vaccinated individual has had two doses of Moderna or Pfizer AND at least 14 days have passed since receiving the second dose OR has had one dose of Johnson and Johnson AND at least 14 days have passed since receiving the vaccine.
A partially-vaccinated individual has had only one dose of Moderna or Pfizer OR had two doses of Moderna or Pfizer BUT fewer than 14 days have passed since receiving the second dose OR has had one dose of Johnson and Johnson BUT fewer than 14 days have passed since receiving the vaccine. Partially-vaccinated and unvaccinated are grouped together in the hospitalization and death numbers as indicated on the chart, but partially vaccinated are excluded from the analysis of case rates comparing unvaccinated and vaccinated.
Q11: What is age adjustment and why is it helpful for comparison?
A11: Age adjustment is an analytical method that takes into account differences that the age of the population might have on the health outcome.
- Example: Imagine you want to compare hospitalization rates among those who are vaccinated and unvaccinated. If the majority of vaccinated people are children or young adults, and the majority of unvaccinated are older adults, the data may appear to show that unvaccinated people have higher rates of hospitalization. However, older adults are more likely to experience severe disease and need hospitalization. Therefore, it is unclear if the higher rate is because of vaccination or because of the age composition of the population.
In epidemiology, age is often called a confounder, a factor that obscures the real relationship between two measures. In our example, the age difference between the populations hid the relationship between vaccination and hospitalizations. There are several ways of analyzing the data to avoid interference from a confounder like age. This method is called adjusting for the effect of a confounder. After adjusting for the difference in the ages of the populations, the comparison of rates between the two groups will be an accurate one.
Q12: What are the acronyms associated with COVID-19 variants and what do they mean?
A12: - Variant of Interest (VOI) – Describes a SARS-CoV-2 variant that is newly emerging and the medical and public health importance is not yet known. (Used by: WHO, CDC, CDPH)
- Variant of Concern (VOC) – A SARS-CoV-2 variant that is considered to be more contagious, cause greater illness, and/or impact treatment or vaccine response. (Used by: WHO, CDC, CDPH)
- Variants Under Monitoring (VUM) – SARS-CoV-2 variants with genetic changes that are shown to affect the characteristics of the virus and indicates that it may pose a future risk, but evidence of phenotypic or epidemiological impact is still unclear. These variants require extra monitoring and assessment of the variant needs to be repeated when new evidence is found. (Used by: WHO)
- Variants Being Monitored (VBM) – SARS-CoV-2 variants where data indicates an impact on treatment or have been associated with more severe disease or increased transmission, but are no longer detected or are circulating at very low levels, so they are not a significant and imminent risk to the public. A VOI or a VOC may be downgraded to this list after the variant sustains a significant decrease in the number of cases, both nationally and regionally, over time, or other evidence proves that the variant does not pose significant risk to the public. (Used by: CDC)
- Variant of High Consequence (VOHC) – A SARS-CoV-2 variant where there is strong evidence that preventative measures or medical countermeasures (MCMs) have significantly decreased effectiveness, compared to previous variants. (Used by: CDC)
Q13: If a case does not report their vaccination status, do they get put into the “unvaccinated” group?
A13: If a case DOES NOT report their vaccination status, the county is able to utilize a database to determine this information. The California Immunization Registry contains records for everyone who has received a COVID-19 vaccine. That information is matched with case records in the California Department of Public Health’s disease surveillance system. Cases that do not have a vaccine record in the database would be considered unvaccinated in our calculations.
Q14: Many vaccinated people have gotten COVID-19. How can the case rates be higher in unvaccinated people?
A14: Rates are different from raw numbers because they take into account the size of the population. This allows for direct comparison of cases in different populations. The population of fully vaccinated residents in Napa County is much larger than the population of unvaccinated residents. Because the majority of Napa County residents are fully vaccinated, it is easy to perceive that there are more cases among people who are vaccinated when looking only at case counts. However, calculating rates among vaccinated and unvaccinated residents (two distinct populations) paints a different picture.
- Example:
- 100 vaccinated cases of COVID-19 among 100,000 vaccinated in the County of Napa
- 100/100,000 x 1,000 = 1 case per every 1,000 vaccinated people
- 100 unvaccinated cases of COVID-19 among 40,000 unvaccinated in the County of Napa
- 100/40,000 x 1,000 = 2.5 cases per every 1,000 unvaccinated people
In this simplified example, even though the raw number of cases is the SAME in both groups, COVID-19 is 2.5 times MORE common in the unvaccinated group of people.
Q15: The vaccine coverage for some age groups is reported to be over 100%. How can this be?
A15: To calculate the percent vaccinated among different groups, such as age or race/ethnicity, epidemiologists need to know the total number in the county population (denominator) and those who received vaccines in the same group (numerator). Although the total population of Napa County is available through the US Census Bureau for 2020, the age and race/ethnicity breakdowns have NOT been released as of yet. As a result, our epidemiologists need to rely on estimates provided by organizations, such as the Department of Finance and ESRI. Currently, we use the ESRI’s 2020 age and race/ethnicity estimates for our denominators. Estimates may be an under- or over- representation of the actual numbers, since populations are dynamic, meaning they are always changing, due to birth, death, migration in or out, etc.. Therefore, if vaccine coverage is over 100% in a subgroup, it is an indication that the population for that subgroup is currently an underestimate. As more current population data becomes available, the denominators will be adjusted to reflect the more current data.