Respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infection
in young children worldwide (
1
–
3
). In the United States, RSV infection results in >57,000 hospitalizations and 2 million
outpatient visits each year among children aged <5 years (
3
). Recent studies have highlighted the importance of RSV in adults as well as children
(
4
). CDC reported RSV seasonality nationally, by U.S. Department of Health and Human
Services (HHS) regions* and for the state of Florida, using a new statistical method
that analyzes polymerase chain reaction (PCR) laboratory detections reported to the
National Respiratory and Enteric Virus Surveillance System (NREVSS) (https://www.cdc.gov/surveillance/nrevss/index.html).
Nationally, across three RSV seasons, lasting from the week ending July 5, 2014 through
July 1, 2017, the median RSV onset occurred at week 41 (mid-October), and lasted 31
weeks until week 18 (early May). The median national peak occurred at week 5 (early
February). Using these new methods, RSV season circulation patterns differed from
those reported from previous seasons (
5
). Health care providers and public health officials use RSV circulation data to guide
diagnostic testing and to time the administration of RSV immunoprophylaxis for populations
at high risk for severe respiratory illness (
6
). With several vaccines and other immunoprophlyaxis products in development, estimates
of RSV circulation are also important to the design of clinical trials and future
vaccine effectiveness studies.
Participating clinical and public health laboratories voluntarily report the number
of aggregate and positive RSV tests to NREVSS each week. In previous years, the RSV
season was defined by consecutive weeks when RSV antigen-based tests exceeded 10%
positivity (
5
); however, since 2008, laboratories have shifted away from antigen-based RSV testing,
and since 2014 the majority of tests and RSV detections among consistently reporting
laboratories are determined by PCR (
7
). From July through the following June of 2014–15, 2015–16 and 2016–17, approximately
56%, 62%, and 72% of RSV detections, respectively, were reported by PCR methods. To
account for these observed changes in testing practice and to more accurately reflect
recent circulation patterns, only results from PCR detection methods are included
in this report.
The method that consistently captured the highest percentage of PCR detections for
retrospectively characterizing RSV seasons was determined to be the retrospective
slope 10 (RS10) method (
7
). This method uses a centered 5-week moving average of RSV detections normalized
to a season peak of 1,000 detections. The season onset was defined as the second of
2 consecutive weeks when the slope, or normalized 5-week moving average of RSV detections
between subsequent weeks, exceeded 10. The season offset was the last week when the
standardized (normalized) detections exceeded the standardized detections at onset.
The peak was the week with the most standardized detections. The season duration was
the inclusive weeks between onset and offset.
Because patterns of weekly RSV circulation in Florida are different from regional
and national patterns, Florida data are reported separately from other national data.
RSV circulation patterns also appear to differ for Hawaii compared with other states
in Region 9 based on limited antigen testing. Therefore, onset, offset, peak, and
duration were summarized using the median of the three seasons nationally (with and
without Florida and Hawaii), by HHS region, and for Florida. There are an insufficient
number of Hawaii laboratories consistently reporting PCR data to present the state
data separately with confidence. Laboratories were included in the analysis if they
consistently conducted PCR testing, as defined by the following criteria: 1) reported
RSV PCR testing results for ≥30 weeks during the 12-month NREVSS surveillance year
and 2) averaged ≥10 PCR tests per week during the 52 weeks of the NREVSS season.
†
From the week ending July 5, 2014 through July 1, 2017, there were three distinct
RSV seasons: 2014–15, 2015–16, and 2016–17 (Figure). For each of these seasons, 135,
218, and 244 laboratories, respectively, reported at least 1 week of RSV testing by
PCR to NREVSS. This analysis was limited to 80 (59%), 108 (50%), and 118 (48%) qualifying
laboratories for 2014–15, 2015–16, and 2016–17, respectively (Table 1). The seasons
as determined by the RS10 method captured 98% of reported RSV PCR detections during
the 2014–15 reporting period and 97% of those reported during the 2015–16 and 2016–17
reporting periods.
FIGURE
Respiratory syncytial virus season duration and peak, by U.S. Department of Health
and Human Services (HHS) Region (headquarters),*
,†,§ and in Florida — National Respiratory and Enteric Virus Surveillance System,
United States, July 2014–June 2015 (A), July 2015–June 2016 (B), and July 2016–June
2017 (C)
* Listed by region number and headquarters city. Region 1 (Boston): Connecticut, Maine,
Massachusetts, New Hampshire, Rhode Island, and Vermont; Region 2 (New York): New
Jersey and New York; Region 3 (Philadelphia): Delaware, District of Columbia, Maryland,
Pennsylvania, Virginia, and West Virginia; Region 4 (Atlanta): Alabama, Florida, Georgia,
Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee; Region 5 (Chicago):
Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin; Region 6 (Dallas): Arkansas,
Louisiana, New Mexico, Oklahoma, and Texas; Region 7 (Kansas City): Iowa, Kansas,
Missouri, and Nebraska; Region 8 (Denver): Colorado, Montana, North Dakota, South
Dakota, Utah, and Wyoming; Region 9 (San Francisco): Arizona, California, Hawaii,
and Nevada; Region 10 (Seattle): Alaska, Idaho, Oregon, and Washington. Delaware,
District of Columbia, Idaho, Iowa, Maine, Maryland, Mississippi, Nebraska, New Hampshire,
New Mexico, North Carolina, Rhode Island, Tennessee, and Wyoming did not have laboratories
meeting the inclusion criteria for the 2014–15 season analysis. District of Columbia,
Idaho, Maine, Mississippi, Nebraska, Nevada, New Hampshire, North Carolina, Rhode
Island, Tennessee, and Wyoming did not have laboratories meeting the inclusion criteria
for the 2015–16 season analysis. District of Columbia, Maine, Nevada, New Hampshire,
Rhode Island, Tennessee, and Wyoming did not have laboratories meeting the inclusion
criteria for the 2016–17 season analysis.
† Region 4 (Atlanta) excludes data from Florida.
§ Region 9 (San Francisco) excludes data from Hawaii.
The figure above is a set of three line graphs showing respiratory syncytial virus
season duration and peak, by U.S. Department of Health and Human Services (HHS) Region
(headquarters), and in Florida, during July 2014–June 2015 (graph A), July 2015–June
2016 (graph B), and July 2016–June 2017 (graph C).
TABLE 1
Summary of 2014–15, 2015–16, and 2016–17 respiratory syncytial virus (RSV) seasons,
by U.S. Departments of Health and Human Services (HHS) Region,* and in Florida — National
Respiratory and Enteric Virus Surveillance System, July 2014–June 2017
HHS region (headquarters) or state/RSV season
No. of laboratories reporting
Onset week ending
Peak week ending
Offset week ending
Season duration (wks)
National
2014–15†
80
10/11/2014
02/07/2015
05/09/2015
31
2015–16§
108
10/17/2015
02/13/2016
05/14/2016
31
2016–17¶
118
10/08/2016
01/14/2017
04/29/2017
30
National without Florida and Hawaii
2014–15†
77
10/18/2014
02/07/2015
05/09/2015
30
2015–16§
104
10/24/2015
02/13/2016
05/14/2016
30
2016–17¶
113
10/15/2016
01/14/2017
04/29/2017
29
Region 1 (Boston)
2014–15†
4
10/25/2014
02/14/2015
06/06/2015
33
2015–16§
5
11/21/2015
02/06/2016
05/14/2016
26
2016–17¶
7
11/05/2016
01/14/2017
05/06/2017
27
Region 2 (New York)
2014–15†
6
10/04/2014
01/31/2015
05/02/2015
31
2015–16§
8
10/31/2015
01/02/2016
05/21/2016
30
2016–17¶
7
10/08/2016
01/14/2017
05/13/2017
32
Region 3 (Philadelphia)
2014–15†
5
11/15/2014
01/10/2015
04/25/2015
24
2015–16§
10
11/07/2015
02/06/2016
05/07/2016
27
2016–17¶
9
10/15/2016
01/07/2017
04/29/2017
29
Region 4** (Atlanta)
2014–15†
6
09/06/2014
12/27/2014
05/30/2015
39
2015–16§
7
09/26/2015
12/19/2015
06/04/2016
37
2016–17¶
7
09/17/2016
12/31/2016
04/22/2017
32
Region 5 (Chicago)
2014–15†
22
10/25/2014
02/14/2015
05/16/2015
30
2015–16§
29
10/10/2015
02/20/2016
05/28/2016
34
2016–17¶
28
10/22/2016
01/14/2017
04/22/2017
27
Region 6 (Dallas)
2014–15†
10
10/11/2014
01/10/2015
04/18/2015
28
2015–16§
11
10/10/2015
01/23/2016
05/07/2016
31
2016–17¶
14
10/01/2016
12/31/2016
05/06/2017
32
Region 7 (Kansas City)
2014–15†
3
10/25/2014
02/21/2015
05/16/2015
30
2015–16§
5
11/14/2015
02/27/2016
05/21/2016
34
2016–17¶
7
10/01/2016
02/04/2017
04/22/2017
30
Region 8 (Denver)
2014–15†
7
11/29/2014
02/14/2015
04/25/2015
22
2015–16§
10
12/05/2015
02/20/2016
05/14/2016
24
2016–17¶
11
11/12/2016
02/11/2017
05/06/2017
26
Region 9†† (San Francisco)
2014–15†
9
11/15/2014
02/07/2015
04/11/2015
22
2015–16§
13
12/05/2015
02/13/2016
04/23/2016
21
2016–17¶
14
11/05/2016
01/21/2017
04/15/2017
24
Region 10 (Seattle)
2014–15†
6
11/15/2014
01/31/2015
04/18/2015
23
2015–16§
7
11/21/2015
02/13/2016
05/07/2016
25
2016–17¶
10
11/05/2016
01/21/2017
04/22/2017
25
Florida
2014–15†
2
09/20/2014
12/27/2014
05/09/2015
34
2015–16§
3
09/19/2015
12/19/2015
04/09/2016
30
2016–17¶
4
09/03/2016
12/03/2016
04/22/2017
34
* Listed by region number and headquarters city. Region 1 (Boston): Connecticut, Maine,
Massachusetts, New Hampshire, Rhode Island, and Vermont; Region 2 (New York): New
Jersey and New York; Region 3 (Philadelphia): Delaware, District of Columbia, Maryland,
Pennsylvania, Virginia, and West Virginia; Region 4 (Atlanta): Alabama, Florida, Georgia,
Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee; Region 5 (Chicago):
Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin; Region 6 (Dallas): Arkansas,
Louisiana, New Mexico, Oklahoma, and Texas; Region 7 (Kansas City): Iowa, Kansas,
Missouri, and Nebraska; Region 8 (Denver): Colorado, Montana, North Dakota, South
Dakota, Utah, and Wyoming; Region 9 (San Francisco): Arizona, California, Hawaii,
and Nevada; Region 10 (Seattle): Alaska, Idaho, Oregon, and Washington.
† Delaware, District of Columbia, Idaho, Iowa, Maine, Maryland, Mississippi, Nebraska,
New Hampshire, New Mexico, North Carolina, Rhode Island, Tennessee, and Wyoming did
not have laboratories meeting the inclusion criteria for the 2014–15 season analysis.
§ District of Columbia, Idaho, Maine, Mississippi, Nebraska, Nevada, New Hampshire,
North Carolina, Rhode Island, Tennessee, and Wyoming did not have laboratories meeting
the inclusion criteria for the 2015–16 season analysis.
¶ District of Columbia, Maine, Nevada, New Hampshire, Rhode Island, Tennessee, and
Wyoming did not have laboratories meeting the inclusion criteria for the 2016–17 season
analysis.
** Excludes data from Florida.
†† Excludes data from Hawaii.
Nationally, across the three seasons, the median RSV onset occurred at surveillance
week 41 (mid-October), and lasted 31 weeks until surveillance week 18 (early May)
(Table 2). The median national peak occurred at week 5 (early February). When Florida
and Hawaii are excluded, the national onset occurred 1 week later and the season duration
decreased by 1 week. Median onset for the 10 HHS regions (excluding Florida and Hawaii)
ranged from week 37 to week 48 (mid-September to early December) and offset ranged
from week 15 to week 21 (mid-April to late May) (Figure). The median season peaks
ranged from week 52 to week 7 (late December to mid-February), and the median duration
ranged from 22 to 37 weeks (Table 2). Region 9 had the shortest season (median = 22
weeks), and Region 4 had the longest (37 weeks). The median onset for Florida occurred
at week 37 (mid-September), and the season continued through week 16 (mid-April) (Table
2).
TABLE 2
Summary of 2014–15, 2015–16, and 2016–17 respiratory syncytial virus seasons by median
and range, by U.S. Departments of Health and Human Services (HHS) Region,* and in
Florida — National Respiratory and Enteric Virus Surveillance System, July 2014–June
2017
HHS region or state
2014–2017 season median and range (surveillance week number)
Onset median surveillance week (mo)
Onset range surveillance weeks (mos)
Peak median surveillance week (mo)
Peak range surveillance weeks (mos)
Offset median surveillance week (mo)
Offset range surveillance weeks (mos)
Median duration (wks)
Duration range (wks)
National
41 (mid-Oct)
40–41 (Oct)
5 (early Feb)
2–6 (Jan–Feb)
18 (early May)
17–19 (Apr–May)
31
30–31
National (excluding Florida and Hawaii)
42 (mid-Oct)
41–42 (Oct)
5 (early Feb)
2–6 (Jan–Feb)
18 (early May)
17–19 (Apr–May)
30
29–30
Region 1
44 (late Oct)
43–46 (Oct–Nov)
5 (early Feb)
2–6 (Jan–Feb)
19 (mid-May)
18–22 (May)
27
26–33
Region 2
40 (early Oct)
40–43 (Oct)
2 (mid-Jan)
52–4 (Dec–Jan)
19 (mid-May)
17–20 (Apr–May)
31
30–32
Region 3
44 (late Oct)
41–46 (Oct–Nov)
1 (mid-Jan)
1–5 (Jan–Feb)
17 (late Apr)
16–18 (Apr–May)
27
24–29
Region 4†
37 (mid-Sep)
36–38 (Sep)
52 (late Dec)
50–52 (Dec)
21 (late May)
16–22 (Apr–May)
37
32–39
Region 5
42 (mid-Oct)
40–43 (Oct)
6 (mid-Feb)
2–7 (Jan–Feb)
19 (mid-May)
16–21 (Apr–May)
30
27–34
Region 6
40 (early Oct)
39–41 (Sep–Oct)
1 (mid-Jan)
52–3 (Dec–Jan)
18 (early May)
15–18 (Apr–May)
31
28–32
Region 7
43 (late Oct)
39–45 (Sep–Nov)
7 (mid-Feb)
5–8 (Feb)
19 (mid-May)
16–20 (Apr–May)
30
30–34
Region 8
48 (late Nov)
45–48 (Nov)
6 (mid-Feb)
6–7 (Feb)
18 (early May)
16–19 (Apr–May)
24
22–26
Region 9§
46 (mid-Nov)
44–48 (Nov)
5 (early Feb)
3–6 (Jan–Feb)
15 (mid-Apr)
14–16 (Apr)
22
21–24
Region 10
46 (mid-Nov)
44–46 (Nov)
4 (late Jan)
3–6 (Jan–Feb)
16 (mid-Apr)
15–18 (Apr–May)
25
23–25
Florida
37 (mid-Sep)
35–38 (Aug–Sep)
50 (mid-Dec)
48–52 (Dec)
16 (mid-Apr)
14–18 (Apr–May)
34
30–34
* Listed by region number and headquarters city. Region 1 (Boston): Connecticut, Maine,
Massachusetts, New Hampshire, Rhode Island, and Vermont; Region 2 (New York): New
Jersey and New York; Region 3 (Philadelphia): Delaware, District of Columbia, Maryland,
Pennsylvania, Virginia, and West Virginia; Region 4 (Atlanta): Alabama, Florida, Georgia,
Kentucky, Mississippi, North Carolina, South Carolina, and Tennessee; Region 5 (Chicago):
Illinois, Indiana, Michigan, Minnesota, Ohio, and Wisconsin; Region 6 (Dallas): Arkansas,
Louisiana, New Mexico, Oklahoma, and Texas; Region 7 (Kansas City): Iowa, Kansas,
Missouri, and Nebraska; Region 8 (Denver): Colorado, Montana, North Dakota, South
Dakota, Utah, and Wyoming; Region 9 (San Francisco): Arizona, California, Hawaii,
and Nevada; Region 10 (Seattle): Alaska, Idaho, Oregon, and Washington. Delaware,
District of Columbia, Idaho, Iowa, Maine, Maryland, Mississippi, Nebraska, New Hampshire,
New Mexico, North Carolina, Rhode Island, Tennessee, and Wyoming did not have laboratories
meeting the inclusion criteria for the 2014–15 season analysis. District of Columbia,
Idaho, Maine, Mississippi, Nebraska, Nevada, New Hampshire, North Carolina, Rhode
Island, Tennessee, and Wyoming did not have laboratories meeting the inclusion criteria
for the 2015–16 season analysis. District of Columbia, Maine, Nevada, New Hampshire,
Rhode Island, Tennessee, and Wyoming did not have laboratories meeting the inclusion
criteria for the 2016–17 season analysis.
† Excludes data from Florida.
§ Excludes data from Hawaii.
Discussion
The national RSV season onsets and offsets reported here occurred in different surveillance
weeks than those reported in previous seasons (
5
). Using PCR data reported to NREVSS, onsets for the 2014–15, 2015–16, and 2016–17
seasons occurred approximately 2 weeks earlier than did those for the 2012–13 and
2013–2014 seasons (early to mid-October versus late October to early November), which
were determined using antigen data; similarly, offsets occurred approximately 4 weeks
later (late April to early May versus late March). These differences largely reflect
the adoption of a statistical method that identifies a consistent inflection point
in weekly RSV detections, rather than a threshold of weekly positivity influenced
heavily by the volume of tests performed (
7
). The differences inherent in evaluating PCR tests, many of which detect several
viral respiratory pathogens, compared with RSV antigen tests, that exclusively detect
RSV, necessitated the adoption of a new statistical method to capture a consistently
high proportion of RSV detections within the defined season (
7
). This change in methodology has resulted in a relative lengthening of the RSV seasons.
Using antigen-based methods, in past years Florida has been observed to have an earlier
onset than other states in the country (
8
). However, using the RS10 method, this earlier onset was not consistently observed.
This report included fewer consistently reporting laboratories in Florida compared
with previous seasons, and the observed patterns might not represent the entire state.
Previous limited antigen-based testing shows that seasonality in Hawaii might differ
from that in other states in Region 9, but too few laboratories have consistently
reported PCR data during the analysis period to present these data separately (https://www.cdc.gov/surveillance/nrevss/rsv/state.html#HI).
Many factors might influence national, regional, and county-level RSV activity, including
social and demographic factors, population density, pollution, and climate (
8
–
10
).
NREVSS surveillance data reflect recent circulation patterns of RSV and might inform
policy decisions regarding administration of palivizumab for immunoprophylaxis. Palivizumab
is a monoclonal antibody recommended by the American Academy of Pediatrics for administration
during the RSV season to infants at high risk and young children likely to benefit
from immunoprophylaxis, based on their gestational age at birth and the presence of
certain underlying medical conditions during the RSV season (
6
).
§
In addition, RSV seasonality data might inform the timing of clinical trials for several
RSV vaccines and immunoprophlyaxis products in development, as well as the evaluation
of product effectiveness after licensure. As testing methods and practices continue
to evolve, CDC might further refine the approach to ascertaining RSV seasons.
The findings in this report are subject to at least four limitations. First, reporting
to NREVSS is voluntary, and analysis is limited to consistently reporting laboratories,
which might not fully represent local and regional circulation. Second, low RSV circulation
might not be captured within the NREVSS onset and offset, although at least 97% of
detections were accounted for using the RS10 method. Third, this report only includes
PCR detections. Although this represents a majority of detections among consistent
reporters, 28%–44% of detections are by antigen methods. Finally, although the number
of positive detections is dependent upon the number of tests ordered, the RS10 method
minimizes this bias by normalizing the detections. Despite these limitations, NREVSS
provides useful information to clinicians regarding RSV circulation and to researchers
designing clinical trials for vaccines and immunoprophylaxis products under development.
The RS10 method used here captures a high proportion of RSV PCR detections for retrospectively
determining RSV seasonality, but cannot be used to determine seasonal onset and offset
in real time, and can only be employed after the season ends. Alternative statistical
methods, including the tenfold baseline or 3% threshold methods (
7
) might be used to determine seasonality in real time or near real time. Timely NREVSS
data and updates of RSV activity at the national, regional, and state levels are published
online weekly at https://www.cdc.gov/surveillance/nrevss. Surveillance data collected
by state and local health departments might more accurately describe local RSV circulation
trends.
Summary
What is already known about this topic?
For most of the United States, the respiratory syncytial virus (RSV) season lasts
from fall through spring but varies from year to year and by geographic region.
What is added by this report?
This report uses a new statistical method that analyzes polymerase chain reaction
laboratory detections reported to the National Respiratory and Enteric Virus Surveillance
System (NREVSS) to determine RSV seasonality nationally and by region for three recent
seasons (2014–2017). Nationally, lasting from the week ending July 5, 2014 through
July 1, 2017, the median RSV onset occurred at week 41 (mid-October), and lasted 31
weeks until week 18 (early May). The median national peak occurred at week 5 (early
February). Onsets for the 2014–17 seasons occurred approximately 2 weeks earlier than
did those for the 2012–2014 seasons (early to mid-October versus late October to early
November), which were determined using antigen data.
What are the implications for public health practice?
RSV seasonality data can guide diagnostic testing and inform policy decisions regarding
administration of currently available immunoprophylaxis products, when indicated,
and the timing of clinical trials and future evaluations of vaccines and immunoprophylaxis
products currently under development.