---
title: "COVID-19 and the 2019-2020 NHANES Cycle"
output: rmarkdown::html_vignette
vignette: >
  %\VignetteIndexEntry{COVID-19 and the 2019-2020 NHANES Cycle}
  %\VignetteEngine{knitr::rmarkdown}
  %\VignetteEncoding{UTF-8}
---

```{r}
#| label: setup
#| include: false

knitr::opts_chunk$set(
  collapse = TRUE,
  eval = FALSE,
  comment = "#>"
)
```

## Overview

The 2019-2020 NHANES survey cycle (suffix K) is **not included** in the datasets provided by this package. This vignette explains why and what it means for your analyses.

## What Happened

NHANES data collection operates on a continuous two-year cycle. The 2019-2020 cycle began on schedule but was suspended in March 2020 due to the COVID-19 pandemic. Only partial data were collected before fieldwork stopped.

The CDC determined that the incomplete 2019-2020 data could not be treated as a standard NHANES cycle. Instead, the agency released it separately as a "pre-pandemic" dataset and combined some components with the subsequent 2021-2023 cycle.

## How This Affects Dataset Naming

NHANES cycles use alphabetical suffixes: `DEMO` (1999-2000), `DEMO_B` (2001-2002), `DEMO_C` (2003-2004), and so on through `DEMO_J` (2017-2018). The 2019-2020 cycle would have been suffix K.

Because this cycle is excluded, datasets skip from suffix J directly to suffix L (2021-2023):

| Suffix | Cycle | Status |
|--------|-------|--------|
| J | 2017-2018 | Included |
| K | 2019-2020 | **Excluded** |
| L | 2021-2023 | Included |

When you load any dataset with `read_nhanes()`, the `year` column will show values from 1999 through 2017, then jump to 2021. There is no `year == 2019` in any dataset.

## Why We Exclude It

1. **Incomplete data collection.** The CDC suspended fieldwork partway through the cycle, so the sample is not representative of the U.S. population in the way that complete NHANES cycles are.
2. **Non-standard release format.** The CDC published these data with different naming conventions and file structures than standard NHANES cycles, which complicates automated processing.
3. **Analytical validity.** The CDC explicitly advises against combining the partial 2019-2020 data with standard two-year cycles for most analyses.

## CDC Documentation

For full details on the 2019-2020 data release and the CDC's guidance on its use, see:

- [NHANES 2019-2020 Overview](https://wwwn.cdc.gov/nchs/nhanes/continuousnhanes/default.aspx?BeginYear=2019)
- [NCHS Data Brief on the pandemic's impact on NHANES](https://www.cdc.gov/nchs/nhanes/)

## Future Plans

We plan to incorporate the 2019-2020 pre-pandemic data in a future release. This requires careful handling of the non-standard naming conventions and clear documentation so users understand the limitations of that cycle. When available, these data will be clearly flagged so they are not inadvertently mixed with standard cycles.

## What to Do in the Meantime

If your analysis requires 2019-2020 data, you can access the pre-pandemic files directly from the CDC using the [**nhanesA**](https://cran.r-project.org/package=nhanesA) package:

```{r}
#| label: direct-access
#| eval: false

library(nhanesA)

# Access 2019-2020 pre-pandemic demographics
demo_k <- nhanesA::nhanes("P_DEMO")
```

Note that the CDC uses a `P_` prefix (rather than the `_K` suffix) for some of these pre-pandemic tables. Consult the CDC documentation linked above for the correct table names.
