You would think that August would bring a lull in the work of U.S. influenza epidemiologists. But August is when we get some of the most important information that predicts what our winter flu season will look like. And the projections are a little scary this year.
The best predictors of North American influenza in our winter is Australian influenza during our summer. Normally, influenza season in Australia starts in April and runs through October, corresponding with winter in the Southern Hemisphere. What happens with influenza in Australia usually fairly closely matches what happens later in the year in the United States. Thus, by examining the epidemiological data from the Australian Department of Health’s Influenza Surveillance, we can predict when influenza cases will start to be seen, what age groups will be affected, what serotypes will be predominant, and what severity will occur here in the United States and Canada.
Recent U.S. influenza seasons
Over the past 3 influenza seasons, we have seen an inverse relationship between COVID cases and influenza. One of the primary reasons for fewer influenza cases when COVID cases increase is social distancing and mask-wearing to prevent COVID. It turns out that these measures help prevent COVID but they are even more effective to help prevent influenza. We can see that effect in the 2019-20, 2020-21, and 2021-22 influenza seasons.
The graph above shows seven previous influenza seasons in the United States. The 2019-20 influenza season (green line) started off quite severe with sustained high numbers of cases from December through March. The onset of the COVID pandemic in the United States in March 2020 marked the closure of schools, work from home initiatives, and public masking. This coincided with a precipitous fall in influenza-like infections at the end of March.
The 2020-21 influenza season (pink line) was the mildest in recent history with only a small peak in cases of influenza-like infections in November and December. At this time, social distancing and masking were more ubiquitous and the COVID vaccines were not yet widely available. It was not until the summer of 2021 that influenza-like infections began to rise – this was a time when COVID vaccines were widely available and it was generally believed that the end of the COVID pandemic was in sight. Consequently, mask mandates were discontinued, children returned to schools, and workers returned to their workplaces. This created conditions that allowed influenza to have a summer rebound.
The 2021-22 season is in red with red triangles. It peaked in December, much earlier than usual. This coincides with the rise in case numbers of the Omicron variant of COVID that caused people to resume masking and social distancing in December. Once these measures to prevent the spread of COVID went back into effect in December 2021, the frequency of influenza-like infections fell.
The exceptional influenza season was the H1N1 outbreak in 2009-10 when cases began to increase in August and peaked in September and October. This represented an unusually early influenza season that caught physicians off-guard. Making matters worse, this particular H1N1 strain had not circulated for decades and was not predicted to appear that season with the result that it was not covered by that season’s flu shots. These factors together resulted in an unusually large number of cases and large numbers of deaths, particularly among younger people who had no natural immunity to H1N1.
What we are learning from Australia
When will influenza season start?
In the last several years, the influenza season in the U.S. has mirrored the influenza season in Australia that occurs earlier in the year. So, what is Australia telling us this year? First, we are likely to see influenza cases start to increase earlier than normal this season. The graph below shows the last several seasons of positive influenza testing in Australia.
The current influenza season is in red. It began much earlier than in past years and also peaked much earlier. Cases began to rise in late April which corresponds to late October in the Northern Hemisphere. Cases peaked in late May in Australia which corresponds to late November in the U.S. By late July, the Australian influenza season was pretty much over – this would correspond to late January in the United States and Canada. So based on these data, we should expect to see influenza cases start to increase in October 2022 with peak numbers in November and December 2022.
How severe will influenza be this year?
Hospitalization data from Australia predicts that this will be an average year with respect to influenza severity. The graph below shows the number of influenza hospitalizations in Australia over the past several seasons. The current season is in red with hospitalizations mimicking the case number graph above. Hospitalizations began to increase in April and were back to baseline by late July.
Based on this data, in the United States, we should expect influenza-related emergency department visits and hospitalizations to peak in November and December 2022.
What ages will be most affected?
A unique finding during the current Australian influenza season has been the propensity to affect children. The graph below shows the number of laboratory-confirmed influenza cases by age.
The largest case rates have been in people under age 20. This would predict that U.S. pediatricians will be seeing more influenza than U.S. internists this season.
Will the influenza vaccine cover it?
The vast majority of cases of influenza in Australia were influenza A with unusually few cases of influenza B as shown in the graph below.
The seasonal influenza vaccines in Australia this year included the following serotypes:
Egg-based quadrivalent influenza vaccines:
- A/Victoria/2570/2019 (H1N1)pdm09-like virus;
- A/Darwin/9/2021 (H3N2)-like virus;
- B/Austria/1359417/2021-like (B/Victoria lineage) virus; and
- B/Phuket/3073/2013-like (B/Yamagata lineage) virus.
Cell-based quadrivalent influenza vaccines:
- A/Wisconsin/588/2019 (H1N1)pdm09-like virus;
- A/Darwin/6/2021 (H3N2)-like virus;
- B/Austria/1359417/2021 (B/Victoria lineage)-like virus; and
- B/Phuket/3073/2013 (B/Yamagata lineage)-like virus.
Although it is still too early to be confident of Australian vaccine effectiveness, we can look at whether the strains seen during the flu season corresponded to the strains covered by the influenza vaccines. In all, 97.4% of influenza A (H1N1) isolates were antigenically similar to the vaccine components. 93.2% of influenza A (H3N2) isolates were antigenically similar to the corresponding vaccine components. And all of the influenza B isolates were similar to the corresponding vaccine components. The U.S. quadrivalent influenza vaccine for the 2022-23 season has identical components to the egg-based quadrivalent influenza vaccine used in Australia. Therefore, it is likely that this season’s flu shots will cover the strains of influenza that we are likely to see in North America.
What we should do in the U.S.
Based on the Australian experience, there are several steps that we should take to prepare ourselves for the 2022-23 influenza season:
- Start vaccinating early. It takes about 2 weeks for immunity to develop after a flu shot. Therefore, we should insure that most Americans get vaccinated in September this year if case numbers begin to rise in October as anticipated. If cases peak in late November, as expected, then people who wait until December or January to get vaccinated will have waited too long.
- Target kids for vaccination. With children being disproportionately affected by influenza in Australia, it is likely that we will see the same trend in the U.S., particularly as schools return to in-person classes.
- Prepare for a surge of hospitalizations in November and December. Normally, this is a low-census period for medical admissions in American hospitals. It is also a time when many people get elective surgeries over the winter holidays and before the end of the calendar year to take advantage of annual insurance deductibles. If the early influenza peak occurs as expected, we may need to institute routine pre-op influenza testing for elective surgeries much as was done with COVID testing during the worst of the COVID pandemic.
- Anticipate the effect of Thanksgiving travel. Thanksgiving and Christmas holidays are times when many Americans travel to be with family. The Australian influenza season predicts that U.S. influenza cases may be peaking around Thanksgiving. This could result in holiday travel accelerating influenza spread this year.
No one can predict the influenza season with 100% accuracy. But if historical trends follow, then the U.S. will likely experience a similar season as Australia. Given that most Americans are starting to relax as the COVID-19 case numbers fall, we could be especially vulnerable to influenza this year, particularly if it comes early and preferentially affects children as expected.
August 10, 2022