This Time It’s Different
Tags: Covid 19, Second Wave, Ireland, Data Science

In Ireland, we are well into the second coronavirus wave, along with the rest of Europe, but it is surprising to see just how different this wave appears to be, when compared with the first wave. Despite the fact that both waves have been similar in duration, and have produced a comparable number of confirmed cases, they differ in terms of the age profile and severity of cases, because of the actions we have taken.

Wave 1 was dominated by older and more severe cases. Wave 2 cases skew much younger, and as a result cases have been far less severe, but not only because of this, because severity has decreased across all age groups.

Even though our new behaviours — social distancing, mask wearing, good hand hygiene — are apparently not sufficient to stall transmission, they do appear to limit disease severity, regardless of age.

If true then wave 2 cases may not lead to the hospitalisations and fatality numbers we saw in wave 1, but only if we continue to observe these behaviours, and only if we can continue to shield the most vulnerable.

It does not mean we can be complacent about rising cases in wave 2, just because they appear to be less severe, but it might mean that restrictions can be more measured and less draconian.

Waves 1 & 2

The graph below shows the daily cases since mid-March, highlighting wave 1 (red) and wave 2 (blue), and showing the 7-day rolling average of these daily cases and the corresponding test positivity rate. Wave 1 lasted for about 75 days and produced more than 24,000 confirmed cases with a median age of almost 48 years-old. To date we have been in wave 2 for 72 days, with over 18,000 cases, but we have yet to find its peak, and cases have a median age of 45 years-old.

The decline from the peak of wave 1 took 45 days — or 60% of the duration of wave 1 — during the equivalent of a Level 5 lockdown that began a full month before the peak. If our current wave follows a similar pattern then a similar lockdown today would see cases peaking in 3 weeks time, at a considerably higher level, which means the decline could take longer than wave 1, taking us beyond Christmas before cases fall to low levels once more. Whether this is a pessimistic prediction or not, it looks likely that wave 2 will end up being significantly larger and longer than wave 1, at least in terms of cases.

That being said, there are many differences between the waves that are not evident by looking at cases alone, particularly in terms of the severity of the disease. Compared to wave 2, test positivity rates were much higher in wave 1, and testing volume was much lower, so it is likely that we are finding many more mild cases in wave 2 compared with wave 1. If so, then we should find a decrease in disease severity. We consider this in the next section by comparing the waves in terms of their hospitalisation and fatality rates.

How do the Waves Differ?

In the graphs below we have aligned the timing of waves 1 and 2 to facilitate a more direct comparison of cases, hospitalisations, and other indicators of disease severity; the dashed vertical line in each plot marks the peak of daily cases in wave 1 for use as a reference.

As mentioned earlier we can see that wave 1 accounted for about 55% of all cases so far, compared to 41% for wave 2. However, despite these similar case numbers, >6 times as many healthcare workers became ill in wave 1. Is this wave 2 reduction due to newly improved infection controls in healthcare settings? Or is it due to fewer hospitalisations of cases, thereby reducing the exposure of healthcare workers to the most severe cases?

We can see that hospitalisations and ICU admissions are also different this time around. The steep growth in admissions in wave 1 is notably absent from wave 2 and, perhaps not surprisingly, the relative share of hospitalisations and ICU admissions between the two waves, exhibits a similar pattern to the infection rate of healthcare workers. This is consistent with the idea that the lower rate of infection among healthcare workers is due to lower hospital/ICU admission rates, and the reduced exposure to the severe cases that this entails.

The profile of deaths from the virus has been even more different in wave 2. In fact, so far wave 2 has only accounted for 72 deaths (3% of all COVID-19 deaths to date) compared with more than 1,600 deaths in wave 1. This adds further weight to the notion that today’s cases are qualitatively different from wave 1’s cases in terms of their severity and lethality.

Why do the Waves Differ?

Before we leap to this conclusion we need to acknowledge that wave 2 is still in progress and, as such, we do not yet have a complete view of its numbers. Since we have not reached its peak, we do not know what the future holds, especially since we are told that hospitalisations and deaths can lag case confirmation by a considerable margin. The worry is that an explosion in hospital admissions and deaths will follow in due course.

In this regard, it is worth noting that hospital and ICU admissions peaked in wave 1 before cases peaked. Deaths peaked about 10 days after cases, but grew they steadily in the weeks before this. Even if cases are being detected earlier in wave 2, leading to a greater lag between case confirmation and hospitalisation/death, would we not have seen some evidence of the type of hospitalisation/fatality growth by now? After all wave 2 has been with us for almost the same length of time as the entirety of wave 1. Perhaps not, especially if the most at-risk groups have not yet participated in wave 2 because they have been shielding. The point is that we should be reluctant to assume that cases will remain mild throughout wave 2.

Also, this time around we don’t want our healthcare system to become over-run with COVID-19, so we should not have to require a repeat of wave 1 hospitalisation levels to become worried. In other words, to preserve a working healthcare system we need to ensure COVID-19 cases continue produce a much lower healthcare system burden. The low hospital burden in wave 2 provides some hope that we will be able to cope better this time, and there are in fact some good reasons why the lower disease severity may be a genuine feature of wave 2 cases:

Improvements in testing and tracing mean that we are finding many more mild cases in wave 2 — the same cases were unknown to us in wave 1 because we didnt have the testing capacity to locate them — and more severe cases are being found faster and treated earlier. Social distancing and mask wearing may help to lower the infectious dose, which may in turn lead to less sever illness. At-risk groups are more likely to be shielding during wave 2 compared with the start of wave 1— this was why hospitalisations peaked early in wave 1 — and we know that the median age of cases is trending lower in wave 2.

Age Matters

The graph below shows the number of cases across various age groups for wave 1 (the red bars) and wave 2 (the blue bars); the number of cases have been normalised per 100,000 people in each age group.

Cases by Age Group

Wave 1 was dominated by older cases and had relatively few younger cases (<25 years-old). In contrast, wave 2 has many more cases from the 15–24 year-old age group and far fewer from the 65+ group. In general, excepting the younger age groups, we can see that the relative number of cases in wave 2 decreases with age as people moderate their behaviour and in line with their perceived level of risk.

My own instinct is that the difference in the younger age groups may bean artefact of testing, in part at least. Younger groups experience milder illness and we didn't have enough testing capability in wave 1 to find them, as we struggled with the many more severe cases, but we do now in wave 2. Even still the low incidence among younger kids now that schools are open, is striking. Kids in the 5–14 age group have the fewest cases per capita even though they are circulating in large groups, mixing with older family members, and less likely to wear masks. They are the least ‘shielded’ and most exposed amongst us, but they are the least likely to become infected; indeed a recent report has highlighted that the coronavirus infection rate in schools is running at about a third of the rate in the wider community.

If the difference between younger groups is somewhat related to testing, then the difference in the 65+ age group between waves 1 and 2 is probably not, and it is more likely to be a genuine difference. There are far fewer older cases because any older people have been shielding or otherwise limiting their close-contacts.

Hospitalisation Rate by Age Group

Hospitalisation rates are often used as an indicator of disease severity and they exhibit a similar pattern in waves 1 and 2, in that the hospitalisation rate increases with age; older cases are more at risk of severe illness. However, the actual rate of hospitalisation is much lower in wave 2 for all age groups, compared with wave 1. For example, over 25% of cases from the 65+ age group were hospitalised in wave 1, compared with just 15% in wave 2. This relative reduction in hospitalisation risk is even greater for younger age groups where the current risk of hospitalisation is now typically much less than 5%.

Perhaps wave 1 cases were more likely to be hospitalised simply out of an abundance of caution as healthcare professionals were still learning about the virus? If so, then the rate of hospitalisation from wave 1 will be less well correlated with disease severity, although this hospitalisation policy seems somewhat unlikely give the priority in wave 1 to preserve hospital capacity for the most severe cases.

We should also recognise that the hospitalisation rate does not provide a complete account of disease severity. One of the most pernicious features of COVID-19 is so-called ‘Long COVID-19’, in which debilitating symptoms continue to plague sufferers for months. It is not clear whether or how this aspect of the disease relates to hospitalisations.

Case Fatality Rate by Age Group

In any event, the case fatality rate has also declined significantly so far in wave 2, compared to where it ended up in wave 1. Fatalities for this virus have always been a feature of older cases; even in wave 1 fatalities among cases younger groups (<55 years-old) were about 0.2% (1 in 500) and in wave 2 it is currently around 0.04% (1 in 2,500) for these groups. Unfortunately, about 20% of the cases from the 65+ year-old group died in wave 1, but the good news is that even for this high-risk age group the fatality rate appears to have fallen significantly in wave 2; so far less than 5% of wave 2 cases from this age group have died. So far…

Once again we need to be careful about how we interpret these data. It could be that the 65+ year-olds who have been exposed recently are becoming infected because they are less anxious about shielding since they have fewer co-morbidities than the general population in this age group; they feel more confident about their ability to deal with the disease and so are willing to shield less. In other words the recently infected 65+ year-olds as less at-risk than the 65+ year-old group as a whole. This would be sufficient to explain the lower fatality rate among those 65+ year-olds in wave 2. But if a greater proportion of this group becomes infected then fatality rates could rise quickly again.

We would we well advised to interpret these data as indications that our behaviours are protecting us, rather than as any sign that the virus has weakened. Still, the lower severity/fatality rates are real and, with all the talk of vaccines and treatments, if someone had told us that there was a way to reduce the fatality rate of this disease by 70%-90%, then I bet we would have viewed this as a major win. This is what the fatality figures from wave 2 suggest today — even among the most at-risk age groups — as long as we continue to find case early, and behave in a way that limits transmission and the infectious dose, and as long as we continue to protect and shield the most vulnerable members of our communities.

Conclusions

There is growing evidence that, compared to the first wave of the coronavirus, the second wave will produce a greater number of cases, take longer to reach a peak, and that its cases will decline more slowly. The current numbers suggest that the virus exhibits a strong seasonality effect and winter-living is likely to facilitate virus transmission, all other things being equal.

The good news is that, so far, wave 2 has skewed towards younger cases, and for this and other reasons, its cases have been less severe and far less lethal than we saw in wave 1. The reasons other than age include the behaviours that we have learned to adopt in order to live with this virus more safely. Social distancing, good hand hygiene, and mask wearing help to limit the infectious dose even when we do become infected, while comprehensive testing and contract tracing identifies cases earlier, and shielding protects the most at-risk among us. All of this combined with the selfless dedication of our healthcare workers, and no doubt some improvements in treatment, have contributed to a greatly reduced disease severity profile this time around, at least so far.

Cases are less severe but their are now more abundant. Unfortunately Level 3 restrictions have not worked to stall case growth. They started well in Dublin, where the R-value did fall to 1, at least for a few days, but it has been rising consistently again over the last week or so. And while transmission rates have slowed in most counties, in many they have not yet started to decline. Level 3, it seems, is simply not powerful enough to turn things around in time. But, while our actions have not been enough to stall case growth, they have been sufficient to blunt the severity of this disease, and without these behaviours we would surely be right back to where we were in wave 1, with overloaded hospitals and a heavy death toll.

This is why we can not afford to lose heart or to become complacent. We’ve already helped to achieve the critical goal of reducing disease severity. Now we need to go further and work to bring cases back down — with a little help from Level 4+ it seems — and once we get cases to more sustainable levels, assuming disease severity remains where it is today, then we can have another shot at living with the virus, by containing transmission through less draconian measures. Here’s hoping!