Vaccine side effects and predicting surges in flu and covid

Family members who got the latest covid booster vaccine that targets the omicron strain all reported side effects of fever, chills, and feeling vaguely lousy for a period that lasted 24 to 48 hours. I had scheduled to get mine last Sunday and the day before I met a neighbor on my walk who said that that was the first day she had been outside because she had had a terrible reaction to the vaccine that had knocked her out for three days. As a result, I cleared my calendar for the three days after the shot was to be given, and made all the preparations to be house-bound and possibly bed-bound for that period.

And then … nothing happened. I had no side effects at all. The pharmacist who gave me both the covid booster and the flu shot at the same time said that since I chose to have them on the same arm, that it might be sore. But even that did not happen. The only thing I did out of my normal behavior was drink plenty of fluids, which is what the CDC recommends to alleviate side effects..

It is the case that side effects vary from person to person and clearly I was among the fortunate ones. But while the variation in reactions is well known, the reasons are not. I recall that when the first covid vaccines were being given, people would compare their side effects and there was a theory floating around that having bad side effects meant that your immune system was working properly. That always seemed a little glib to me, but it did help to make the people with bad side effects feel better.

We are being warned that the coming winter flu season might be quite severe and that we should get the flu shot. How do they predict this? They look to Australia, which just had its worst flu season in five years and “which experiences winter ahead of the U.S.”, because the summer and winter seasons there are six months off from those in the northern hemisphere.

Researchers and modelers often look to the southern hemisphere, which experiences its flu season first — typically from May to October — to predict how the season will look in the U.S., and experts tell ABC News we should take warning from Australia.

“We often look to Australia and the southern hemisphere as a signal of what we may expect,” said Dr. John Brownstein, an epidemiologist at Boston Children’s Hospital and an ABC News contributor. “Obviously, it’s not a perfect 1-to-1 match but, more often than not, the severity of the flu season in Australia is a good correlate of what we might expect, and it helps us prepare.”

But why do we see Australia as a leading indicator? Why can’t the reverse be true, that the flu season in the US is a predictor of what will happen in Australia six months later? What do Australians use as leading indicators? Since the US had a very mild flu season last winter and Australia just had its worst flu season in five years, then that direction of flow would not work. But is that just a post facto conclusion based on what happened this time? Or is it always the case that Australia is a leading indicator? And if so why? I have not been able to get an answer to that question.

Another pattern that has been observed is that the covid numbers in Europe are also used as a leading indicator for what happens in the US and the recent signs of a surge there are being viewed with concern here.

I am still wearing my mask in indoor public places. As to when I might stop wearing it, I am thinking of using as a benchmark when the deaths from covid drop to about the level of deaths from flu. Some reports suggest that we have already reached that point, though that conclusion seems less than certain.

Of course, one could argue that even then, it might make sense to continue wearing the mask in order to avoid both the flu and covid. I have got so used to wearing it that it is only mildly irksome.


  1. invivoMark says

    With the caveat that my understanding could be slightly outdated, seasonal flu typically originates in south or southeast Asia. This is thought to be in part due to close proximity of humans with farm animals, since flu can undergo rapid antigenic shift by mixing the genomes of two different variants (the genome of influenza is broken into 8 separate segments). An animal infected with two different variants can therefore produce a unique variant that is a mix of the two.

    I have not heard of Australia as being a predictor of US flu seasons. A 2010 study of flu genomics seemed to indicate that the opposite was the case -- that there is more transmission of flu from the US to Australia than the other way around. Study:

    The past two flu seasons have been mild because of COVID precautions, but as people have been giving up on mask-wearing and social distancing, that is likely why Australia’s flu season is back to a high severity.

  2. anat says

    I also had no side effects from the latest updated booster (with flu vaccine in the same shoulder). My plan at the moment is to keep masking in indoor public spaces forever. I haven’t had any respiratory illness of any kind since February 2018 and I like it this way.

  3. billseymour says

    I, too, had no reaction to any of my four Pfizer jabs, not even a sore arm.

    I usually notice a sore arm around bedtime on the day of my annual flu shot; but it’s not seriously painful, just a little uncomfortable; and it’s gone by morning.  That happened this year as well.

    If I got any sniffling or sneezing from any of the shots, I wouldn’t have made that connection at the time because being allergic to just about every kind of pollen that exists is the more likely explanation.

    I, too, continue to wear an N95 whenever I leave my apartment and will probably continue to do so, even if COVID prevalence gets down to flu level, because I’m used to it now and it doesn’t really bother me.

  4. dro says

    A couple of ABC Australia items.

    Dr McCauley works with the WHO to identify new strains of influenza and potential new components for upcoming vaccines.

    Australia is renowned for its robust influenza surveillance, so it’s no surprise new strains have been identified here. “You’re always looking to see what’s been seen in the southern hemisphere,” Dr McCauley said. “There is very good surveillance done in Australia. So, if you actually look at the composition of influenza vaccines, the number that come from places like Darwin, Brisbane and South Australia is quite striking.”’s-flu-season/13971052
    Dr Andy Pekosz is a virologist at the Johns Hopkins Bloomberg School of Public Health in Baltimore, Maryland and has told ABC Newsradio what’s happening in Australia could help predict what’s to come in the Northern Hemisphere.

  5. says

    It’s not dying from Covid that scares me, it’s long Covid.

    Anyway, my previous shots had all been Pfizer and except for the sore arm I was fine. This time it was Moderna so I was prepared to feel oogy but it was the same, sore arm and nothing else.

  6. prl says


    But why do we see Australia as a leading indicator? Why can’t the reverse be true, that the flu season in the US is a predictor of what will happen in Australia six months later? What do Australians use as leading indicators?

    According to the Australian Influenza Vaccine Committee, the composition if the flu vaccine for the 2023 flu season in Australia (vaccines will be made available around March/April in preparation for the southern winter), the considerations were:

    The Australian Influenza Vaccine Committee (AIVC) met to recommend the composition of the influenza virus vaccines for Australia in 2023. At this meeting, the expert committee reviewed and evaluated epidemiology, antigenic and genetic data of recent influenza isolates circulating in Australia and the southern hemisphere. Serological responses to the 2021-2022 vaccines, and the availability of candidate vaccines viruses and reagents were also reviewed by the Committee. [Dated 6 Oct 2022]

    So it looks like the decision is largely driven by what variants were in circulation in the period leading up to the meeting (i.e. winter/early spring in the southern hemisphere).

    Before I checked, I thought it was largely driven by which variants were in circulation in the northern hemisphere flu season immediately before the southern hemisphere flu season, but since the AIVC recommendation lists the variants to be included in the Australian 2033 vaccine, it looks like the decision is made before the main variants in circulation in the northern hemisphere flu season are known.

  7. prl says

    As for COVID vaccine reactions, I had (for me compared to other vaccines I remember) an unusually bad reaction to my first AstraZeneca vaccine, about 4 days of feeling generally somewhat lethargic and slightly nauseous (enough to put me off a couple of meals, but never actually vomited), along with the usual day or two’s tenderness at the injection site.

    My second AstraZeneca vaccine and the two subesquent Pfizer boosters I had resulted in no more than a day or two’s tenderness at the injection site.

  8. flex says

    My boss indicated he felt the Flu shot more than his COVID booster, having gotten them in different arms.

    Because of that, I looked up the volume of fluid injected for each:

    Flu shot = 0.5ml
    Pfizer booster = 0.3ml
    Moderna booster = 0.25ml

    So, aside from any effects from the vaccines themselves, the volume injected is different which may result in a different amount of injection site pain. I realize that this is minor compared to the reactions some people have with the contents of the flu or COVID shots, but I thought it was interesting. I’ve had no issues with any shots, but I’m getting my flu and booster in three weeks so we’ll see what this next time brings.

  9. billseymour says

    flex @8:  where did you find that data?

    I’d like to find out whether the 0.5ml is the same for both versions of the flu shot.  I’m an old fart who gets the “stronger” version, whatever that means.  I wonder whether I’m getting a higher concentration of “active ingredients” or just more of the same vaccine.

  10. flex says

    billseymour @9,

    I just googled it. I have never asked, or known, that there were different versions of the flu shot, I’ve just taken what was available at clinic.

    However, this chart from the CDC looks like it might answer your question:

    If you are getting the high-dose vaccine, HD-IIV4, it looks like the volume of liquid in the shot is 0.7ml, with the HA (hemagglutinin) count of 60ug in the 0.7ml dose. Being a young man in my fifties ( 😉 ), I’m likely to be getting the standard ccIIV4 0.5ml does with 15ug HA. Although, my boss appears to have gotten the RIV4, which is a 0.5ml shot with 45ug of HA. That seems to be a relatively new vaccine, but popular, so I might end up with that one.

    I note that some shots show a volume of 5ml, but these are apparently designed for one of those jet injectors, not a needle, which penetrates the skin with a high pressure jet of fluid. I haven’t seen one of those since my time in the military where thousands of Airmen were processed in a day with multiple vaccines. I’d rather not have that type of injection again, it felt like there was a golf-ball under my skin.

  11. flex says

    Sorry, to answer your question, the answer is yes. You are likely getting more fluid in the shot as well as a stronger dose.

    40% more fluid, but 400% of the dose.

  12. billseymour says


    … one of those jet injectors … which penetrates the skin with a high pressure jet of fluid.  I haven’t seen one of those since my time in the military where thousands of Airmen were processed in a day with multiple vaccines.

    Yep.  I went through Air Force basic training in ’66 and one day got three of those in one arm and two in the other.

  13. brightmoon says

    I got my Covid booster on Monday. Sore arm but not a bad as usual. I usually can’t lift my arm to above my ribs for about 2 days

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