Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Influenza A Virus in Swine Surveillance

Fiscal Year 2021 Quarterly Report

Surveillance Summary for First Quarter Fiscal Year 2021: October 1 to 

December 31, 2020

Report Summary1

• This report covers the first quarter (Q1) of fiscal year (FY) 2021 from October 1 through 

December 31, 2020.

• Where relevant, the report also includes previous years’ data for historical perspective. 

• The report provides data from both national and regional levels.

• In Q1 of FY2021, there were 632 samples submitted for influenza A virus (IAV) surveillance in 

swine from 564 accessions. 

• H1N1 was the predominant subtype reported in USDA data in Q1 FY2021.

• Over the past 8 quarters, H1N2 was the main subtype in Regions 1, 3 and 5, H1N1 in Region 2 

and Region 4 isolated H3N2 most frequently.

• Limited accessions from a region can skew data and lead to misinterpretation. Therefore, less 

inference can be applied to results from Regions 3, 4, and 5.

• All IAV-S submissions are voluntary and based on clinical case submissions to veterinary 

diagnostic labs. These data are not a statistically representative sampling of the U.S. swine 

population.

• Due to the voluntary nature of this surveillance, the information in this report cannot be used to 

determine regional and/or national incidence, prevalence, or other epidemiological measures, 

but it may help identify IAV-S trends. 

Introduction

This report, based on data received into the database as of February 16, 2021, provides a brief update 

on the status of national surveillance for IAV in swine for producers, swine practitioners, diagnosticians 

and the public. Summaries in this report may differ from those provided in past reports due to the 

regular addition of data from participating laboratories. Reporting months are based on the month the 

sample was collected. The IAV-S surveillance program is voluntary and, as a result, the accessions and 

samples submitted represent a subset of the swine population. Submitted samples should only be 

collected from animals displaying influenza-like illness. When the submitter does not report relevant 

information, data are recorded as “unknown.” Due to its voluntary nature, this surveillance system does

not entirely represent the total U.S. domestic swine population. Therefore, the data cannot be used to 

1 In November 2016, VS modernized the process that prepares and stages laboratory results data for reporting. Consequently, 

VS recognizes there is a small difference in previously reported summary numbers for IAV-S surveillance. The results in this 

report reflect updated and corrected numbers achieved with the modernized data process.
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

determine IAV-S prevalence or other epidemiologic measures in the swine population. However, the 

data may help identify trends of influenza in swine. 

A laboratory accession generally represents a set of samples collected at a single premises on a single 

day and received at the laboratory. While a nasal swab or lung tissue sample represents a single animal 

within the herd, a single oral fluid sample may represent one to two pens of animals in a herd. A positive 

sample status is based on the screening real-time reverse transcriptase polymerase chain reaction (rRTPCR) on one or more samples within the accession. The subtype result is based on rRT-PCR-based 

subtyping assays. Virus isolation (VI) and sequencing in the NAHLN labs are only attempted on rRT-PCR 

positives meeting criteria with sequences deposited into GenBank, the public sequence database. On a 

monthly basis, USDA NVSL also performs whole genomic sequencing (WGS) on a selected subset of virus 

isolates received into the repository through the surveillance program and deposits those sequences

into Genbank. On a quarterly basis, a phylogenetic analysis is performed by ARS influenza researchers; 

phylogenetic analyses are based on all successful USDA surveillance sequencing results deposited into 

GenBank, the public sequence database. 

Program Updates

Information on IAV-S and the IAV-S surveillance program, as well as previous IAV-S quarterly reports, are 

found at:

https://www.aphis.usda.gov/aphis/ourfocus/animalhealth/animal-disease-information/swine-diseaseinformation/influenza-a-virus

The focus of IAV-S surveillance remains on acquiring and analyzing contemporary viruses from sick swine 

for ongoing genetic studies. The National Animal Health Laboratory Network (NAHLN) has several 

submission options to ensure that unusual viruses identified by methods other than standardized

NAHLN testing processes can be submitted into the program. An updated version of the IAV-S NAHLN 

testing guidelines and instruction sheet can be found at:

• Algorithm: 

https://www.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/appendix_c_tes

ting_guidelines.pdf

• Instructions: 

https://www-author.aphis.usda.gov/animal_health/animal_dis_spec/swine/downloads/iav-salgorithm-instructions.pdf

IAV-S Surveillance Objectives

USDA’s National Surveillance Plan for Swine Influenza Virus in Pigs (July 2010) describes the current 

surveillance system for IAV in swine in detail. The surveillance objectives are to: 

1. Monitor genetic evolution of endemic IAV in swine to better understand endemic and emerging 

influenza virus ecology; 

2. Make influenza isolates from swine available for research and establish a data management system to 

facilitate genetic analysis of these isolates and related information; and 

3. Select proper isolates for the development of relevant diagnostic reagents, updated diagnostic assays, 

and vaccine seed stock products. 
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Objective 1. Monitoring Genetic Evolution of Endemic IAV in Swine to Better 

Understand Endemic and Emerging Influenza Virus Ecology

Objective 1 is met through the submission of diagnostic laboratory samples to the surveillance system, 

collection of the viruses that are isolated from the samples, and analysis of the hemagglutinin (HA) and 

neuraminidase (NA) sequences that are generated at the NAHLN laboratories. Each month, selected

viruses undergo whole genomic sequencing by the National Veterinary Services Laboratories (NVSL). 

Phylogenic analysis of the genetic sequences submitted through the surveillance program is provided 

through an interagency agreement with the USDA’s Agricultural Research Service (ARS) National Animal 

Disease Center (NADC). 

National Surveillance Data Summary

From FY2010 through FY2015, the total number of accessions and samples submitted increased.

Changes initiated in FY2016 resulted in decreased laboratory accessions and samples, but a higher 

percentage of accessions resulting in a virus isolate that can be sequenced and analyzed. In Q1 FY2021, 

632 samples have been tested from 564 accessions (Figure 1). Figure 2 shows the overall trends in total 

accessions, rRT-PCR and VI positive accessions, and subtyped accessions. Figure 3 shows the number of 

subtype detections in Q1 FY2021. The total number of samples subtyped was 375, including H1N1 

(n=143), H1N2 (n=113), H3N2 (n=107), H3N1 (n=1), and mixed (n=11).

 

Figure 1. Number of IAV laboratory accessions and samples tested in swine FY2017 through Q1 FY2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Figure 2. Accessions submitted, subtyped accessions, rRT-PCR positive accessions, and virus isolation positive accessions over 

time with trend lines for IAV-S, FY2017 through Q1 FY2021

Figure 3. Number of IAV-S subtype detections in Q1 FY2021

Figure 4 breaks down accessions by rRT-PCR subtype from FY2017 through Q1 FY2021. H1N1 was the 

predominant subtype detected in 2012, 2013, 2014, 2018, 2020 and Q1 FY2021. H1N2 was detected 

most often in 2011, 2015, 2016, 2017, and 2019. 

Figure 5 displays the number of times VI was conducted in blue, the number of times VI was conducted

and was positive in purple, and the number of viral isolates submitted to GenBank in green. Since the 

implementation of the June 2016 modifications to the program, almost all VIs attempted now yield a 

virus and the sequences are submitted to Genbank for analysis.
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Figure 4. Breakdown of accessions by subtype rRT-PCR from FY2017 through Q1 FY2021

Figure 5. Number of virus isolations attempted, positive virus isolations, and GenBank submissions from FY2017 through Q1

FY2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Laboratory accessions were evaluated by age-class for the first quarter. The most common subtype 

isolated among the nursery and grow/finish classes were H1N1. Among the suckling age class, H1N2 was 

the predominant isolated subtype. The sow/boar class had limited testing, with two isolates of H1N1

and one H1N2 isolates reported. Among isolates for which the age class was unknown or not recorded, 

H1N1 and H1N2 were the predominant subtypes (Table 1). Excluding specimen types with less than 10 

percent of total submissions, lung samples were the most successful at providing positive virus isolation

and submission to GenBank (Table 2).

Table 1. Number of positive accessions tested for IAV-S by age class and viral subtype, Q1 FY2021

Age Class (group)

Number 

of H1N1

Number 

of H1N2

Number of 

H3N1

Number 

of H3N2

Number 

of Mixed

Suckling 22 25 0 21 2

Nursery 56 40 0 49 4

Sow/Boar 2 1 0 0 0

Not Recorded/Unknown 17 17 0 11 2

Grow/Finish 46 30 1 26 3

Table 2. Number of positive accessions* tested for IAV-S by specimen type and by viral subtype, Q1 FY2021

Specimen 

Type 

(group)

Number of 

accessions 

with 

subtype 

reported

Percent of 

subtyped 

accessions 

with 

positive 

virus 

isolation

Number 

of H1N1

Number 

of H1N2

Number 

of H3N1

Number 

of H3N2

Number 

of 

Mixed

Number of 

samples 

sequenced 

and sent to 

GenBank

Lung 342 91% 135 101 1 95 10 306

Nasal or 

Nasal Swab 20 70% 7 5 0 8 0 14

Oral Fluids 13 92% 1 7 0 4 1 10

*Accessions may include samples with multiple specimen types. In these cases, individual accessions are counted 

in more than one specimen type category. 
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Regional surveillance data

In this section, we present data in five different regions (Figure 6). These regions are based on former USDA 

administrative districts only and do not represent specific industry distribution. Submissions are voluntary, 

as is any identifying information accompanying the submission (except the State of animal origin), and 

therefore no sampling strategies can be applied to the regions.

Figure 6. A map of the regions for national IAV-S surveillance

Summary of Regional Data from ARS

Table 3. Summary of predominant subtypes by region in a 1 year window from Jan 2020 through Dec 

2020.

Region Total number HA/NA subtype

1 117 H3N2 (H3-Cluster IV-A / N2-2002)

H1N2 (H1-Delta2 / N2-1998) 

H1N1 (H1-Gamma / N1-Classical)

2 779 Most diversity of all regions

H1N1 (H1-Gamma / N1-Classical)

H1N2 (H1-Delta2 / N2-1998)

H3N2 (H3-Cluster IV-A / N2-2002) 

3 81 H1N1 (H1-Pandemic / H1-Pandemic)

H3N2 (H3-2010.1 / N2-2002)

H1N2 (H1-Delta2 / N2-1998)

4 63 H3N2 (H3-2010.1 / N2-2002) 

H1N1 (H1-Gamma / N1-Classical)

H1N1 (H1-Pandemic / N1-Pandemic)

5 18 Low participation

H3N2 (H3-2010.1 / N2-2002)

H1N1 (H1-Gamma / N1-Classical)

H1N1 (H1-Pandemic / H1-Pandemic)

Most Predominant HA/NA phylo-types overall: 

H1N1 (H1-Gamma / N1-Classical) (n=244)

H1N2 (H1-Delta2 / N2-1998) (n=149)

H3N2 (H3-Cluster IV-A / N2-2002) (n=132)

H3N2 (H3-2010.1 / N2-2002) (n=77)
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Figure 7 shows the distribution of rRT-PCR subtyped accessions among the five regions for Q1 FY2019

through Q1 FY2021. Regions 1, 3 and 5 demonstrate H1N2 as the predominant subtype, Region 2

demonstrates H1N1 as predominant and Region 4 isolated H3N2 most frequently. For regions recorded as 

“unknown,” H1N1 and H1N2 were equally the most frequent subtypes.

Figure 7. Distribution of rRT-PCR subtyped accessions among the five regions Q1 FY2019 through Q1 FY2021

Regional phylogenetic analysis

Phylogenetic analysis of sequences from the IAV-S surveillance system

Phylogenetic analysis of gene sequences of IAV in swine is conducted to further examine the genetic 

changes that occur in HA, NA, and Matrix (M) genes of this rapidly changing virus. Through collaboration 

with ARS, a dataset2,3 of 350 isolates with published sequences in GenBank was characterized by 

phylogenetic analysis for the Q1 FY2021 report. This analysis provides information on the genetic diversity 

and evolution patterns of influenza in swine and allows for inferences about population and/or vaccine 

immunity.

2 Participating NAHLN labs included M gene sequencing in their testing until July 2016 because the 2009 H1N1 M gene was the 

predominant circulating gene. 3 The ARS dataset is comprised of IAV-S surveillance isolate sequences that were posted in Genbank. This represents only a subset 

of the complete IAV-S surveillance dataset that includes PCR diagnostic test-based results as well as sequencing results. Therefore, 

ARS dataset results, such as subtype percentages, differ from the complete IAV-S dataset results provided in other sections of this 

report.
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

The following series of bar charts parse the data into an approximately 2-year window by quarters and 

subtypes for each region, followed by charts further describing the phylogenetic clades of H1 and H3 

subtypes. Regional charts depicting the various combinations of HA and NA are in Appendix 1.

Figure 8 demonstrates the four subtypes H1N1, H1N2, H3N1 and H3N2 across the five regions. Regions 1 

and 2 reported the most submissions, with a mixture of mostly H1N1, H1N2, and H3N2. Limited accessions 

from a region can skew data and lead to misinterpretation and therefore, less inference can be applied to 

results from Regions 3, 4, and 5. 

National phylogenetic HA gene information 

HA genes from H1 subtype viruses are classified as alpha, beta, gamma, delta-1, delta-2, or pandemic H1N1 

2009 (H1N1pdm09) phylogenetic clades based on a previously published nomenclature system. Similarly, 

H3 subtype viruses are classified as Cluster IV, Cluster IV-A, Cluster IV-B, Cluster IV-C, Cluster IV-D, Cluster 

IV-E, Cluster IV-F, or human-like. In Q1 FY2021: H1-Gamma was the predominant H1; H1-Pandemic is 

rapidly growing; and there were no H1-Gamma2-beta-like LAIV observed this quarter (Figure 9).

Figure 8. Temporal distribution of Influenza A virus subtype by region for Q2 FY2019 to Q1 FY2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Figure 9. Temporal distribution of H1 phylogenetic clades by region for Q2 FY2019 to Q1 FY2021

In Q1 FY2021: H3-Cluster IV-A was the predominant H3; Unique pairs: H3-2010.1 / N1-MN99-LAIV, H3-

2010.2 / N2-2016, H3-Cluster IV-I / N2-1998, H3-Cluster IV-B / N2-2002; and no H3-Cluster I LAIV were 

observed this quarter (Figure 10).

National phylogenetic NA gene information 

Whole genome sequencing with HA/NA pairs with internal gene lineages were still pending at the time this 

report was created.

Objective 2. Make Influenza Isolates from Swine Available for Research and 

Establish a Data Management System to Facilitate Genetic Analysis of these 

Isolates and Related Information

A primary goal of IAV swine surveillance is to share selected virus isolates obtained through the surveillance 

system with public health, animal health, and academic researchers to facilitate genetic analysis and 

research on viruses of interest. The NVSL Diagnostic Virology Laboratory maintains a repository of the 

viruses submitted into the surveillance system and provides these viruses upon request. 

In Q1 FY2021, the NVSL Diagnostic Virology Laboratory provided 13 isolates to two academic institutions 

and one government requestor. NVSL received 319 isolates into the repository. Table 4 reports the total 

number of virus isolates received into the repository each year from FY2014 through Q1 of FY2021. Table 5

reports the total number of isolates available in the repository by subtype for sharing. 

Figure 10. Temporal distribution of H3 phylogenetic clades by region for Q2 FY2019 to Q1 FY2021
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Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Table 4. Virus isolates received in repository 

Virus isolates in the repository

FY2021 319

FY2020 1,074

FY2019 1,055

FY2018 994

FY2017 844

FY2016 1,046

FY2015 883

FY2014 765

TOTAL TO DATE 6,980

 

 

Objective 3. Select Proper Isolates for Development of Relevant Diagnostic 

Reagents, Updating Diagnostic Assays, and Vaccine Seed Stock Products

USDA makes IAV-S isolates available in the public domain for further research. ARS-NADC conducts 

research on isolates obtained from the repository and sequences generated from the surveillance system. 

Genetic sequencing from the USDA program that is reported to GenBank is available for private 

corporations, government entities, academia, and other scientific community partners for research and 

vaccine strain selection and efficacy testing. NVSL and ARS staff are consulted as subject matter experts 

when necessary. 

Conclusion

The IAV voluntary surveillance system in swine continues to provide insight into the genetic makeup of 

circulating influenza A virus in limited populations of commercial pigs. Genetic information and virus 

isolates are made publicly available for further research and possible vaccine strain selection and efficacy 

testing. Influenza A virus in swine remains a dynamic virus with high levels of genetic variability in the 

hemagglutinin and neuraminidase genes. 

Table 5. Total number of subtyped 

isolates available through repository

Subtyped isolates available through 

repository

H3N2 2,373

H3N1 21

H1N1 3,061

H1N2 2,845

Mixed 302

TOTAL 8,602
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021

Appendix 1. Regional Charts of HA and NA Combinations by Percentage

The following charts present the percentages of combinations of HA and NA on the national and regional scales based on ARS-NADC phylogenetic 

analyses. The results are reported from January 2020 through December 2020. These “heat maps” represent the percentage of combinations by 

using a color gradient where a deeper gradient color represents a greater percentage occurrence for a particular HA-NA combination. HA clusters 

are listed on the left vertical axis of the chart and NA clusters are listed on the bottom horizontal axis. Line up the HA cluster with the 

corresponding NA cluster to determine the occurrence of that particular combination. 
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021
Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1

Influenza A Virus in Swine Surveillance Quarterly Report for Fiscal Year 2021, Quarter 1 March 2021