https://orcid.org/0000-0001-5794-7500
ABSTRACT
Kuwait started immunizing children <2 y age with the 7-valent pneumococcal conjugate vaccine, PCV7 from August 2007. PCV7 was replaced by the 13-valent conjugate vaccine, PCV13 from August 2010. In a previous analysis of the results for the period, August 2010–July 2013 (period II), there was no evidence of serotype-specific protection for invasive disease against the additional six serotypes to PCV7 present in PCV13 (non-PCV7 serotypes) as evidenced by isolation from blood and cerebrospinal fluid in any of the age groups, <2 y, 2–5 y, 6–50 y, 51–65 y, and >65 y and all ages, compared to the pre-vaccination period, August 2003–July 2006 (period I). In the current study, we allowed additional time, August 2013–July 2019 (period III) for better vaccine effect and repeated the analysis. We did not find any significant decrease of invasive disease due to the non-PCV7 serotypes of PCV13 in period III and combined II and III periods compared to period I. However, these comparisons showed significant reductions for four of the six and total serotypes of PCV7, and total serotypes of PCV13. Reduction for total PCV13 serotypes was contributed by serotypes of PCV7. It appears that the six non-PCV7 serotypes in PCV13 do not offer much protection. Some contributory factors for the poor effect of the non-PCV7 serotypes may be related to few cases with underpowered statistical analysis, lack of vaccine coverage data, method of vaccine efficacy analysis based on vaccine serotypes relative to all serotypes and unusual rise in non-typeable isolates post vaccination that would have masked true serotypes.
Introduction
The 23-valent polysaccharide pneumococcal vaccine (PPV23) was introduced in Kuwait in the late 1970s for high-risk individuals aged >2 y and healthy adults aged >65 y.Citation1 The 7-valent conjugate pneumococcal vaccine (PCV7) was introduced in August 2007 in the national immunization program for children aged <2 y, administered at the ages 2, 4 and 6 months with a booster dose at 18 months. The 13-valent conjugate vaccine (PCV13) replaced PCV7 from August 2010 with a catch-up immunization with PCV13 given to children 2–5 y of age appropriately vaccinated for age with PCV7. As childhood vaccinations are compulsory in Kuwait, pneumococcal vaccine coverage is expected to be nearly 100%Citation1 even though no data exist. This assumption is based on a reward and punishment system. Government services including access to schooling (primary schooling is compulsory) is denied to unimmunized children who are eligible for vaccination. PCV7 contains capsular polysaccharides from serotypes 14, 6B, 19F, 18C, 23F, 4 and 9V, while PCV13 has polysaccharide capsules from additional serotypes, 1, 5, 7F, 3, 6A and 19A.Citation2 In our previous study, which included a pre-vaccination period, August 2003 to July 2006, and post-vaccination period, August 2006 to July 2013, we found evidence of vaccine-induced protection against serotypes covered in PCV7, and not against non-PCV7 serotypes in PCV13 for the post-PCV13 period, August 2010 to July 2013. We surmised that not enough time has lapsed to assess protection against invasive disease due to these serotypes after the introduction of PCV13.Citation3 We reassessed immunity against these serotypes after a further period of 6 years from August 2013 until July 2019.
Materials and methods
The study comprised period I (pre-vaccination from August 2003 to July 2006), period II (post-vaccination with PCV13 from August 2010 to July 2013) and period III (post-vaccination with PCV13 from August 2013 to July 2019). Data for period II and part of the data for period III (for 41 months from August 2013 to December 2016) were previously published.Citation3,Citation4 These data were included in the current study for a continuous and relatively large data set and for proper analysis.
Pneumococcal isolates studied were from sterile site specimens – blood and cerebrospinal fluid (CSF). A single isolate per patient was studied. If a patient had isolates from both blood and CSF at the same time, CSF isolate was chosen. The isolates originated from children and adults seeking care at general hospitals, tertiary-care hospitals, and polyclinics throughout Kuwait. The isolates were sent to the Pneumococcal Reference Laboratory at the Faculty of Medicine, Kuwait University, Jabriya, Kuwait, on blood agar culture plate. The isolates were reconfirmed as Streptococcus pneumoniae by positive tests for α-hemolysis on blood agar, optochin susceptibility, and bile solubility. The isolates were stored in blood glycerol broth (2.5 g nutrient broth and 16.8 ml glycerol made up to 95 ml with distilled water and sterilized and supplemented with 5 ml sterile sheep blood) at −70°C for further study. Demographic data of patients including age and gender were recorded. The age groups were stratified into <2 y, 2–5 y, 6–50 y, 51–65 y, and >65 y.
Serotyping of the isolates was performed by Quellung reaction with rabbit polyclonal antisera (Statens Serum Institute) in the Danish chessboard typing system.Citation5
For the current study, the following definitions were used. PCV7 serotypes are 14, 6B, 19F, 18C, 23F, 4 and 9V, and PCV13 serotypes are the PCV7 serotypes plus six additional serotypes, 1, 5, 7F, 3, 6A, and 19A. These six additional serotypes in PCV13 are referred to as PCV13 non-PCV7 serotypes or PNP serotypes. Serotypes not contained in PCV13 are referred to as non-PCV13 serotypes.
The impact of vaccination was assessed by the changes in the distribution of recovered isolates in relation to serotypes represented in the conjugated vaccines.Citation3,Citation6
Data were compared between periods I and III, periods I and II + III, and periods II and III. Chi-square test was used to find out the significance of difference between two groups with a 95% confidence interval (CI). Fisher’s exact test was applied in case the observations were small. A two-tailed probability value P ≤ 0.05 was considered statistically significant.
Results
The distribution of invasive serotypes, which consisted of both vaccine and non-vaccine serotypes, for the different periods is shown in . It appeared to be similar for the different periods. Among children <2 y age, the numbers of PNP serotypes in period II versus period III were: 0 vs 0 (serotype 1), 0 vs 0 (serotype 3), 0 vs 1 (serotype 5), 0 vs 1 (serotype 6A), 0 vs 0 (serotype 7F), and 1 vs 1 (serotype 19A). For children 2–5 y age, the figures for the two periods were: 0 vs 0 (serotype 1), 0 vs 0 (serotype 3), 0 vs 0 (serotype 5), 0 vs 0 (serotype 6A), 0 vs 0 (serotype 7 F), and 2 vs 0 (serotype 19A). Some non-vaccine serotypes appeared to increase in the post-vaccine periods. The serotypes that had >5 isolates in the two combined post-vaccine periods were: 8, 9N, 12F, 15A, 15B, 17F, 23A and 33D. Non-typeable isolates had the largest increase in the post-vaccination periods.
Table 1. Serotypes of invasive pneumococci in different age groups during pre-vaccination and postvaccination (with PCV13) periods
Differences in the distribution of vaccine serotypes for <2 y group were compared (). There was a significant reduction in the prevalence of total PCV7 serotypes and total PCV13 serotypes in post-vaccination periods III and combined vaccination periods II + III compared to the pre-vaccination period I. However, no significant reductions were found for the individual serotypes. Also, no significant reductions were seen for total PNP serotypes in the comparisons. The prevalence of total PCV7 serotypes was higher than that of PNP serotypes in pre-vaccination period though the difference was not statistically significant (5/9 versus 1/9, P = 0.13).
Table 2. Differences in the distribution of vaccine serotypes in <2 y age group
Distribution of vaccine serotypes for 2 y to >65 y age group is shown in . For PCV7 serotypes, there were significant decreases in the prevalence for serotypes 9V, 14, 19F, 23F, and total serotypes in the post-vaccination periods (III, II+III) compared to the pre-vaccination period (I). For PNP serotypes, no significant decrease was found in the post-vaccination periods either for individual serotypes or total serotypes. For total PCV13 serotypes, there was a significant decrease in the two post-vaccination periods (III, II+III). As PCV13 provides questionable protection against serotype 3Citation7, the prevalences of total PNP serotypes and total PCV13 serotypes were compared between periods I and III disregarding the number of serotype 3 isolates. For total PNP serotypes, the figures were: 39/54 (72.22%) vs 21/109 (19.3%) (P = <0.001 with CI, 2.47–5.70).
Table 3. Differences in the distribution of vaccine serotypes in 2 y->65 y age group
Discussion
Several studies have shown the immediate effect of vaccination on reducing IPD due to the serotypes included in the vaccine.Citation8–10To account for the delayed effect of vaccination,Citation11 we analyzed the results for the second period, August 2013 until July 2019, and for the total period, August 2010 to July 2019. IPD due to total PCV7 serotypes and total PCV13 serotypes was significantly reduced in both periods. Even though, reductions in IPD due to PNP serotypes were seen in both post-vaccination periods, the differences were not statistically significant. If significant reduction is not seen in age groups up to 5 y, it is unlikely that we will see significant reductions in older age groups. We should be cautious about the true number of invasive serotypes studied. The reference laboratory is dependent on the individual laboratories to send their isolates. Even though there is a ministry of health decree to send all isolates to the reference laboratory, it is possible that every isolate may not be reaching the laboratory. There is no surveillance system to monitor how many isolates are missed on an yearly basis.
Different protective effects for various PNP serotypes of PCV13 were seen in different studies. In the studies in the USA,Citation12 varying levels of protection (58−93%) was seen against serotypes 1, 7F, and 19A. In the UK, effectiveness was 90% for the PCV7 serotypes, but only 73% for four of the six additional serotypes of PCV13.Citation13 In another study in the UK, 5 years after the introduction of PCV13, there was a significant reduction in the invasive disease due to serotypes 1 and 7F, but not 3 and 19A in the late period compared to early period.Citation14 In a study in Denmark, a significant change in IPD incidence due to serotype 19A, but no change for serotypes 1 and 3 was seen.Citation15 A study in Israel found an increase of 47% IPD due to PNP serotypes initially followed by a 79% reduction.Citation10 In a combined Saudi Arabian and Kuwaiti study (which included data for 41 months which is also included in the present study), the effect of vaccination was analyzed for age groups, <2 y, 2−5 y, 51−65 y, >65 y and all ages, by annual rate – number of isolates in a period divided by the number of years in that period. A mixed picture was seen in Kuwait. The annual frequency for PNP serotypes fell for 2−5 y and >65 y age groups but increased for the other two age groups and all ages following PCV13 vaccination. However, in the two age strata studied in Saudi Arabia (0−2 y and 3−5 y), the annual rates for PNP serotypes rose after vaccination. Since the authors did not treat their data statistically, the significance of changes in PNP serotypes could not be assessed. Nevertheless, it appears that PCV13 vaccination did not have much positive effect on IPD due to PNP serotypes.Citation4
The reasons for the apparent low effect of PCV13 on PNP serotypes could be several. 1) Our passive surveillance system may not be reliable which depends on isolates reaching us. This may obscure the difference between true fluctuations due to vaccine effect and that due to a bad surveillance system. 2) Our analysis is dependent on vaccine serotypes relative to all serotypes. Vaccination increases NVTs resulting in a relative decrease of vaccine serotypes even if their absolute numbers do not change. 3) There is a lack of data on vaccine coverage. 4) There is an unusual rise of non-typeable strains post-vaccination. Are these strains not true non-typeable strains but lost their capsule due to improper handling and storage? If proven correct, this could change our calculus on vaccine efficacy. 5) It can be argued that the numbers of PNP serotypes are small and that the statistical analysis is underpowered. However, even with small numbers of PCV7 serotypes causing IPD, there were significant reductions in some serotypes of PCV7 and total PCV7 serotypes and total PCV13 serotypes causing IPD. The reduction in total PCV13 serotypes causing IPD is also contributed by PCV7 serotypes. However, this could be due to longer term effect of PCV7 as it has been studied longer than PCV13. Also, the effectiveness against PCV7 serotypes is expected to be higher than the effectiveness against PNP serotypes.Citation12,Citation16Protection against serotypes can be correlated by serum antibody levels against the capsular polysaccharide antigens. An aggregated level of 0.35 μg/ml of antibody is estimated to be the threshold level for protection against IPD.Citation17Some studies found low serum antibody concentrations against serotypes 4, 6B, and 19F after PCV10 or PCV13 vaccination.Citation13,Citation18The antibody levels of people after vaccination have never been studied in Kuwait. It is possible that vaccination is not producing the required levels of antibodies to afford protection against PNP serotypes. Host genetic factors such as expression of IL8 and IL1B may also be involved in antibody production.Citation19Several studies have found that PCV13 had either no or little effect on IPD due to serotype 3.Citation14,Citation15,Citation20,Citation21Elimination of serotype 3 from our calculation improved the protection afforded by PNP serotypes. It is believed that serotype 3 is highly virulent.Citation22 Capsular and non-capsular factors and genetic background of the organism are contributory to virulence of this serotype.Citation23 Vaccine failure has also been shown with serotypes 19A.Citation24,Citation25 Studying the genetics and virulence of PNP serotypes from Kuwait may shed light on the problem.
Rise of non-vaccine serotypes (NVT) after the introduction of PCV13 has been observed in many countriesCitation26–28 and the main NVT serotypes are 8, 9N, 15A, and 23B.Citation29 We have also seen an increase of some of these NVT in our study. This can add some complexity in our analysis. Vaccine efficacy calculation is based on the number of vaccine serotypes relative to total number of serotypes. Therefore, a rise in number of NVTs because of vaccination will depress the relative numbers of vaccine serotypes even when the absolute numbers do not change. This caveat must be taken into consideration as discussed above. Also, there was a relatively large increase of non-typeable strains post vaccination. This was striking for <2 y and >65 y age groups, in which the numbers jumped from 0% to 11% of isolates in period I to more than one-third of the isolates in period III (). Could this be due to the loss of capsule because of less optimal handling and storage of isolates? These strains need to be characterized for their capsular genes.Citation30
It appears that the additional six serotypes included in PCV7 to make it PCV13, do not appear to offer much protection to combat IPD due to these serotypes in Kuwait. The same appears to be true for Saudi Arabia. On the contrary, PCV7 component of PCV13 continues to be highly effective in Kuwait. Higher valency vaccines are being evaluated which include PCV15 which has two additional serotypes (22F, 33F) to PCV13Citation31 and PCV20 which has five additional serotypes (8, 10A, 11A, 12F, 15B, 22F, and 33F) to PCV13.Citation32 PCV20 may benefit Kuwait as three serotypes included in this vaccine, 8, 12F, and 15B are on the rise in Kuwait post PCV13 vaccination.
It is important to monitor the effectiveness of a vaccine introduced in a country or region to assess the benefits of vaccination. It appears that a “one-size-fits- all” approach for pneumococcal vaccination, where a single vaccine is used in all countries, irrespective of circulation of different serotypes may not work in all countries. Tailor-made vaccines depending on the dominant serotypes causing IPD in a country or region will have to be considered.
Author contributions
EM collected the bacterial isolates and patient data. SS did the laboratory work. MJA wrote the manuscript. EM and SS approved the final version.
Disclosure of potential conflicts of interest
No potential conflicts of interest were disclosed.
Additional information
Funding
References
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