diff --git a/src/api/services/ingestion.go b/src/api/services/ingestion.go
index 9c040c4..c00fff2 100644
--- a/src/api/services/ingestion.go
+++ b/src/api/services/ingestion.go
@@ -24,6 +24,7 @@ import (
 	"os/exec"
 	"path"
 	"regexp"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -370,7 +371,11 @@ func (i *IngestionService) ProcessVcf(
 	defer gr.Close()
 
 	scanner := bufio.NewScanner(gr)
+
 	var contigs []string // To collect contigs as defined in VCF header
+	var contigMutex = sync.RWMutex{}
+	var preMadeContigIndices bool = false
+
 	var discoveredHeaders bool = false
 	var headers []string
 	headerSampleIds := make(map[int]string)
@@ -401,10 +406,12 @@ func (i *IngestionService) ProcessVcf(
 					fmt.Printf("Error: got invalid contig header '%s' (matches: %v)\n", line, matches)
 				} else {
 					// Valid
+					// Currently this isn't parallelized, but we use the mutex just in case
+					contigMutex.Lock()
 					contigs = append(contigs, matches[2])
+					contigMutex.Unlock()
 				}
-			}
-			if line[0:6] == "#CHROM" {
+			} else if line[0:6] == "#CHROM" {
 				// Split the string by tabs
 				headers = strings.Split(line, "\t")
 
@@ -421,31 +428,19 @@ func (i *IngestionService) ProcessVcf(
 				// --> create required indices (one per contig) with mappings to ensure ES types are consistent and
 				//     mitigate issues we've encountered with e.g., SIGNATURE, where a date field was detected for
 				//     info.value.
+				contigMutex.Lock()
 				fmt.Printf("Got %d contigs: %v\n", len(contigs), contigs)
 				for _, c := range contigs {
-					var client = i.ElasticsearchClient
-					var contigIndex = variantIndexName(c)
-
-					res, err := client.Indices.Exists([]string{contigIndex})
-					if res.StatusCode == 404 {
-						mappings, _ := json.Marshal(indexes.VARIANT_INDEX_MAPPING)
-						res, _ := client.Indices.Create(
-							contigIndex,
-							client.Indices.Create.WithBody(strings.NewReader(fmt.Sprintf(`{"mappings": %s}`, mappings))),
-						)
-
-						fmt.Printf("Creating contig index %s - got response: %s\n", c, res.String())
-					} else if err != nil {
-						// The actual check didn't work properly (e.g., couldn't contact ES).
-						fmt.Printf("Contig index %s existence-check got error: %s\n", c, err)
-					} else {
-						// The check worked and the index already exists, so we shouldn't try to recreate it.
-						fmt.Printf("Contig index %s already exists; skipping creation\n", c)
-					}
+					i.MakeVariantIndex(c)
+				}
+				contigMutex.Unlock()
+				if len(contigs) > 0 {
+					// flag to prevent trying to make contig indices on-the-fly during ingestion
+					preMadeContigIndices = true
 				}
 
 				discoveredHeaders = true
-				fmt.Println("Found the headers: ", headers)
+				fmt.Printf("Found %d headers: %v\n", len(headers), headers)
 			}
 			continue
 		}
@@ -478,9 +473,9 @@ func (i *IngestionService) ProcessVcf(
 			rowWg.Add(len(rowComponents))
 
 			for rowIndex, rowComponent := range rowComponents {
-				go func(i int, rc string, rwg *sync.WaitGroup) {
+				go func(h int, rc string, rwg *sync.WaitGroup) {
 					defer rwg.Done()
-					key := strings.ToLower(strings.TrimSpace(strings.Replace(headers[i], "#", "", -1)))
+					key := strings.ToLower(strings.TrimSpace(strings.Replace(headers[h], "#", "", -1)))
 					value := strings.TrimSpace(rc)
 
 					// if not a vcf header, assume it's a sampleId header
@@ -491,6 +486,18 @@ func (i *IngestionService) ProcessVcf(
 							// Strip out all non-numeric characters
 							value = strings.ReplaceAll(value, "chr", "")
 
+							if !preMadeContigIndices {
+								// If we have to make contig indices on the fly, check if we haven't created the contig yet.
+								// If we haven't, create the index and add it to the contigs slice.
+								// // A bit janky - O(n) lookup every time for whether contig exists
+								contigMutex.Lock()
+								if !slices.Contains(contigs, value) {
+									i.MakeVariantIndex(value)
+									contigs = append(contigs, value)
+								}
+								contigMutex.Unlock()
+							}
+
 							// ems if value is valid chromosome
 							if chromosome.IsValidHumanChromosome(value) {
 								tmpVariantMapMutex.Lock()
@@ -908,6 +915,28 @@ func (i *IngestionService) FilenameAlreadyRunning(filename string) bool {
 	return false
 }
 
+func (i *IngestionService) MakeVariantIndex(c string) {
+	var client = i.ElasticsearchClient
+	var contigIndex = variantIndexName(c)
+
+	res, err := client.Indices.Exists([]string{contigIndex})
+	if res.StatusCode == 404 {
+		mappings, _ := json.Marshal(indexes.VARIANT_INDEX_MAPPING)
+		res, _ := client.Indices.Create(
+			contigIndex,
+			client.Indices.Create.WithBody(strings.NewReader(fmt.Sprintf(`{"mappings": %s}`, mappings))),
+		)
+
+		fmt.Printf("Creating contig index %s - got response: %s\n", c, res.String())
+	} else if err != nil {
+		// The actual check didn't work properly (e.g., couldn't contact ES).
+		fmt.Printf("Contig index %s existence-check got error: %s\n", c, err)
+	} else {
+		// The check worked and the index already exists, so we shouldn't try to recreate it.
+		fmt.Printf("Contig index %s already exists; skipping creation\n", c)
+	}
+}
+
 func variantIndexName(contig string) string {
 	return fmt.Sprintf("variants-%s", strings.ToLower(contig))
 }