Viral Mystery: Analysing Singapore’s 271 ‘Breakthrough’ Covid-19 Cases
Visual/data analysis of 271 people in Singapore who tested positive for Covid-19 despite having one or two shots of the vaccine. This is not a medical study.
The Covid-19 pandemic has confounded medical experts and policy makers alike since it began in 2020. More than a year on, they are grappling with an emerging new mystery involving “breakthrough cases” — people who caught Covid-19 despite being vaccinated.
No vaccine is 100% effective so it’s not a surprise that some vaccinated individuals would still test positive. But it remains unclear how these breakthrough infections occur, whether demographic or environmental factors play a bigger role, and how soon a booster shot is needed in the face of new and more contagious Covid-19 variants.
This post takes a look at 271 such breakthrough Covid-19 cases in Singapore, via publicly released data from the Ministry of Health’s website. I began tracking these cases in May 2021, and stopped on June 29 when the ministry stopped providing detailed case notes on each new infection in Singapore.
Several caveats before we examine the data:
- This dataset of 271 cases is not a comprehensive tally by any means. It is merely the fullest version I have been able to assemble with publicly available data.
- Some key pieces of medical information are not available — such as the health condition of the breakthrough cases, and the treatment they received to cope with Covid-19, if any was required.
- The number of breakthrough cases is very, very small relative to the number of people who have been vaccinated. As of June 30, a day after details about these breakthrough cases were no longer available, Singapore had fully vaccinated about 2.1 million people, while 1.21 million have received one dose. The 196 fully vaccinated breakthrough cases in this dataset represent just 0.009% of those in Singapore who have received two shots of the Covid-19 vaccine, up to the point when data were available for comparison.
- This is not a medical study (obviously), and I’m neither a medical or vaccine expert. I’ve taken care to not over-interpret the data, and there’ll be constant reminders in this post about the limitations of this dataset/analysis.
Maybe the biggest caveat is that having data is not the same as having answers — a lesson impressed upon many of us throughout the pandemic. Still, I hope this modest analysis would add a little to our understanding of the virus that has reshaped the way we live.
TLDR: RESULTS SUMMARY
Before getting into the details, here are the top line results that I found most interesting and relevant (I’ll be referencing median instead of mean values throughout due to the considerable gap between the extremes of the data points):
- Of the 271 breakthrough cases, 196 (72.3%) involve full vaccinated individuals who have had two shots of the Moderna or Pfizer-BioNTech vaccines, while 75 (27.7%) had only one shot.
- There are only 20 mentions of the Delta or B1617 variant in the case notes, representing just 7.4% of the 271 breakthrough cases. But it is unclear if absence of evidence is evidence of absence in this instance.
- About a third, or 92 of the 271 breakthrough cases, were asymptomatic, while the rest experienced a range of symptoms from fever to runny nose to fatigue.
- For breakthrough cases who were fully vaccinated, the median number of days between their second shot and confirmation of Covid-19 infection is 59 days. For breakthrough cases who only got one shot, the median number of days between their one and only jab and confirmation of Covid-19 infection is just 13 days.
- Men make up 62% of the breakthrough cases, while 149, or 55% of the 271 cases, are aged 50 and above. The median age of the breakthrough cases is 52. Singapore’s vaccination programme focused first on older segments of the population as well as frontline workers. The socio-demographic profile of breakthrough cases could well change in the coming months as younger people are vaccinated in larger numbers. As such, I would avoid jumping to conclusions based on demographic trends at this point, though they are well worth a look.
The dataset and a detailed Jupyter notebook for the charts/analysis are available in my repo for this project. Additional notes about the dataset are at the end of this post.
#1: IS THE DELTA VARIANT TO BLAME?
Let’s first deal with the main question on everyone’s mind — how many of these “breakthrough cases” involve the B1617, or Delta, variant?
The data here is unfortunately patchy. There are only 20 mentions of the Delta or B1617 variant in the case notes (see this data subset here), or about 7.4% of the 271 breakthrough cases in this dataset.
Additionally, the Singapore health ministry had indicated that these 20 people only “tested preliminarily positive” for this particular variant. Results of the confirmatory tests have not been publicly released, to the best of my knowledge. We don’t know either if the other 92.6% of breakthrough cases had been put through further tests for the Delta variant, and if so, what those results might be.
So we have the classic question of whether absence of evidence is evidence of absence. Publicly available data in Singapore can’t answer this question satisfactorily, for now.
#2: “PROTECTION WINDOW”, OR NUMBER OF DAYS BETWEEN VACCINATION AND “BREAKTHROUGH” INFECTION
Covid-19 breakthrough cases are rare relative to the number of people vaccinated. But to the extent that they happen, is there a difference in the “protection window” for those who were fully vaccinated versus those who were partially vaccinated?
As the box-plot chart below shows, the difference is not insignificant. Among the 196 breakthrough cases involving fully vaccinated individuals, the median number of days between their second shot and confirmation of infection is 59 days.
For those who only had one shot and were only partially vaccinated, the median number of days between getting the jab and confirmation of infection is just 13 days.
I’m not a medical or vaccine expert, so I’ll avoid opining on whether the 59-day window seen for fully vaccinated breakthrough cases is within expectations, or something to be concerned about.
Singapore does not officially administer any of the single-dose vaccines, and the case notes aren’t entirely clear as to why 75 of these breakthrough cases only had one shot of the vaccine. From the dataset, we can guess that some of these “single shot” breakthrough cases caught Covid-19 before they could get their second jab. For others, we can only assume that they either didn’t know they had to take a second shot, or forgot about their second appointment, or decided to skip it.
Regardless, the message is quite clear in this instance: Don’t skip the second vaccine shot! And just as importantly, don’t get complacent after your first shot.
#3. ASYMPTOMATIC BREAKTHROUGH CASES Vs THOSE WHO EXPERIENCED SYMPTOMS
Another question on my mind is whether fully vaccinated breakthrough cases are harder to spot, ie, they don’t experience or exhibit any symptoms that might warn them or others that they have been infected.
Overall, about 1-in-3 of the 271 breakthrough cases are asymptomatic — 92 had no symptoms while 179 reported experiencing symptoms like fever, running nose, and fatigue. The side-by-side bar charts below give a more detailed breakdown by comparing the breakdown of symptomatic/asymptomatic cases among fully and partially vaccinated breakthrough cases.
Among the 75 partially vaccinated breakthrough cases, 72% (54 cases) experienced symptoms. Among the 196 fully vaccinated cases, 64% (125 cases) experienced symptoms. In other words, a clear majority of breakthrough cases experienced symptoms, whether they are fully or partially vaccinated.
I’ll leave the experts to explain whether the numbers are higher or lower than expected. From a lay person’s perspective, the message here is again one about staying vigilant: Don’t dismiss the tell-tale symptoms of Covid-19, like fever or runny nose, even if you have been fully or partially vaccinated.
Multiple studies have shown that vaccination will protect you from severe illnesses arising from Covid-19 infection. But that does not make you immune to the mild symptoms, as the chart above illustrates.
4. DEMOGRAPHIC BREAKDOWN OF BREAKTHROUGH CASES
I deliberately left the demographic breakdown of the 271 breakthrough cases to the last for two reasons:
- Singapore started vaccinating its elderly population first, as well as frontline workers with higher risk of exposure to the virus. A different demographic profile of the breakthrough cases might emerge once more of the general population is vaccinated.
- Environmental and/or occupational factors could be more important in some cases. For instance, a considerable number of elderly breakthrough cases in Singapore involved cleaners who worked at the Changi Airport, where a major cluster emerged in May 2021. Did the elderly cleaners get infected on account of their age or working environment?
As such, it is best not to jump to too many conclusions about demographic trends at this point. The overall picture could well look very different when a fuller study is done months down the road.
For what’s its worth, the key demographic takeaways for the 271 breakthrough cases are:
- 167 cases, or 61.6%, involve men, while women make up the remaining 104 cases.
- The median age is 52. The youngest breakthrough case is 12, while the oldest is 93.
- 149, or 55% of the 271 breakthrough cases, are aged 50 and above. This is somewhat expected as Singapore vaccinated its elderly population first, and the trend could well change as a larger number of younger Singaporeans are vaccinated.
- There is no significant gap between the median age for male and female breakthrough cases — 51 and 53 respectively — as the chart below shows:
I’ve created an interactive chart on Flourish if you wish to examine the demographic data in greater detail. It is available here.
5. END NOTE
Given the public policy implications of breakthrough cases and the growing debate about the need for a third booster shot, I think it is still worthwhile to take a closer look at these 271 cases despite the clear limitations of the dataset.
For me, the key takeaway is the importance of staying vigilant, even if you have been partially or fully vaccinated. While very rare, breakthrough infections can occur — and could take place within a period of time that may surprise you.
Among fully vaccinated individuals in this dataset, the median number of days between confirmation of Covid-19 infection and their most recent jab is 59 days. For the partially vaccinated, this median window fell to just 13 days.
A clear majority of breakthrough cases experienced symptoms, whether they were fully or partially vaccinated.
While we wait for fuller studies to explain how breakthrough infections take place, it seems prudent to maintain sensible safe-distancing measures and mask wearing in crowded/public places, even as more people get vaccinated.
This flies in the face of a growing public clamour for such measures to be lifted as vaccination rates rise. But given how many times we’ve been wrong footed by this virus and its growing number of variants, I don’t think this is a risk worth taking at this point.
6. DATASET
The 271 breakthrough cases in this dataset, the first of which was first publicly disclosed on 30th January 2021, is not a comprehensive tally by any means. For instance, I did not include a number of breakthrough cases among residents and staff at MINDSville@Napiri, a home for people with intellectual and developmental disabilities, as the Singapore health ministry only released summary case notes on their vaccination status. No explanation was given as to why only a summary was provided for those cases.
Corrections and revisions to the ministry’s data aren’t easy to track either, so there’s no telling if had been more breakthrough cases than those tallied here. By the time you are reading this, there’ll certainly be more than 271 breakthrough cases in Singapore.
You can download this dataset from the repo or via Dropbox here. If you want to extract other details not in the columns shown here, such as occupation, check out the “Details” column.
Here’s a quick breakdown of the types of data you’ll find in the various columns:
Reference Data
- See columns ‘Case_Number’, ‘Details’, and ‘URL’.
Demographic Data
- See columns ‘Age’, ‘Age Range’, ‘Gender’, and ‘Nationality’.
Vaccination Data
- See columns ‘Date_1st_Vac’, ‘Date_2nd_Vac’, and ‘Vac_Status’. ‘Date_1st_Vac’ and ‘Date_2nd_Vac’ refer respectively to the dates where an individual got his or her first and/or second vaccination shot. Their overall vaccination status (‘Vac_Status’) is indicated as “First Shot Only” or “Full Course”. Singapore’s official Covid-19 vaccination programme uses the Pfizer-BioNTech and Moderna mRNA vaccines, both of which entail a two-dose regime.
Infection/Symptoms Data
- See columns ‘Date_Confirmed’, ‘Date_Onset_Symptoms’, ‘Symptom_Status’, and ‘Cluster’ ‘Date_Confirmed’ refers to the date that an individual officially tested positive for Covid-19, while ‘Date_Onset_Symptoms’ refers to the date when he or she first experienced symptoms linked to the virus. Those with no symptoms are classified as “Asymptomatic” in the “Symptoms_Status” column. Note that the “Clusters” column is incomplete due to the information lag in contract tracing, ie, some cases aren’t linked to a particular cluster at the point of announcement due to time needed for verification.
As always, if you spot mistakes in this or any of my earlier posts, ping me at:
- Twitter: Chua Chin Hon
- LinkedIn: www.linkedin.com/in/chuachinhon
